Is replacing a rear derailleur an easy to do?

yes

https://www.parktool.com/blog/repair-help?area%5B%5D=52

i just did mine yesterday for the first time. it was easy. but as i learned you should expect to make adjustments to the H and L limit screws even if no wheel/cassette is installed. it was my assumption that it would shift fine with no cassette (but not in the correct places), i was wrong about that. because i had my chain off at the time i put the wheel/cassette back in and made the H and L adjustments. after putting on the chain it was very easy to make slight barrel adjustments. the hardest part was getting a compatible RD to replace my old Sora 8 speed.

Don't forget to put the shifter into the smallest cog (most cable payed out) position before attaching the cable to the derailleur. You should also check the condition of the cable and housing and clean/lube/replace as needed. Pay special attention to the last run of housing as it tends to pick up water and crud from the road.

Thanks, I saw several YouTubes, and it looks pretty easy. How do I find out a right rear derailleur is compatible with my bike?

The simplest solution is to replace the derailleur with the same model. If this isn't possible for some reason and you want recommendations for a substitute, you'll need to provide info. Current group including shifters, derailleur, cassette (tooth range) and it won't hurt to say what bike it's on. Basic info. If this is stuff you can't identify, then you shouldn't be messing with it, take it to a shop.

Mine is a 10 speed mountain bike, the cassette is 11-46. I think the model for the rear derailleur is no longer available.

What is the derailleur on it now?

I think I just found the same model. Thanks.

So we need to know what you are currently using or some info on the bike before we can really help you. We don't know anything about your bike at all other than maybe you want to replace the rear derailleur on it so we know it is probably not a single speed but beyond that, that is all we can really glean. If you wanted help figuring this information out you would want to give us info.

Here is how I would write a post on a subject like this if I was somewhat new to the whole thing:
"Hey folks, I have a Specialized Roubaix and the s-piece or speed changer (actually lingo I have heard working at a shop from customers) says Shimano Ultegra and has 11 cogey dealeys at the back. I need a new one, does this one would work: https://bike.shimano.com/en-EU/produ...-R8000-SS.html. Thanks in advance for the help" (note I found that searching 11 ultegra and scrolling down slightly on the Shimano site)

You could also give us pictures of the bike if you had 10 posts or more and as much help as possible and if people asked questions you could answer them as best as you can.

People around here tend to not like being used as AltaVista or Ask Jeeves or Yahoo Answers! when the internet is full of great information even if sometimes less than accurate can usually get you closer or you can in some cases disseminate the bad info somewhat or take that info with what you have and know and then use that to come up with better questions.

Imagine someone came to you with a question and it was so general and broad that you really couldn't do much with it would you want to answer that question or would you get a little peeved. Imagine you saw that often and saw bigger spikes lately like that. You have to help us to help you we don't know what you have and with little to no info it is hard to really give you good info or any info without something ourselves.

Okay, mine is Trek Marlin 7. The rear derailleur is Shimano Deore M5120, long cage.

For what you need I would probably just replace with what you have but if you did want to upgrade or for some really dumb reason downgrade (which I never recommend unless you absolutely have to) you just need to make sure it is the same number of speeds, the correct length cage and the correct cable pull. After 9 speed Shimano road and mountain diverged so you want to stick with XTR, XT, SLX, DEORE... Just make sure you have the correct number of speeds. I am assuming you have a 10 speed looking at Trek's page for the bike so this Shimano XT RD-M786-SGS or Deore RD-M6000-SGS would work fine. The RD-T6000-SGS could probably work but it is designed for their trekking groupset rather than a mountain bike which you have (hence the "T" vs "M") so no clutch which is handy on a 1x MTB. According to Shimano the derailleur you currently have is also 11 speed compatible but I know cable pull is different for 10 and 11 but probably not nearly enough to cause issue if you did go 11 but if you go 11 you just need a shifter and cassette and you are at 11 speeds already.

There is a guy on Youtube that had issues with your bike and that derailleur because of the clutch but I refuse to link to that video because he links to and supports a known knockoff pedal in his video and I don't play that but is easily searched via the googles. However please for the love of Eddy Merckx and all that is Drillium (or holey) do not buy knockoff stuff.

Folks, if you can't offer friendly helpful advice in a thread, simply move on to another thread.

Thx, what is T vs M and clutch?

This is completely unsolicited advice.... but are you sure the RD needs replacing?

Unless it is visually damaged, they tend to last a very long time.

I fell down on my bike and scrap the rear derailleur really hard. I’m thinking about to get an upgrade.

If you had a wreck and the RD is now not shifting well there is an 98% chance that the issue is a bent derailleur hanger, not the RD itself, and a new RD will not work any better.

OTOH, if the RD still works fine, and you just don’t like the the looks of the scrape... it looks like you found what you need.

It’s good to know. Thanks.

Don't buy the chain breaker tool from Harbor Freight (I love Harbor Freight btw). It's too big for bike chains, even though it says it works on them.

Get yourself a good bike chain breaker and it's pretty easy after that.

Why are you changing your derailleur?

T=Trekking (or sometimes Touring)
M=Mountain

and a clutch is a device to help keep your derailleur from bouncing and keep the chain on. It is also a decent rock band from Maryland worth checking out.

"I am assuming you have a 10 speed looking at Trek's page for the bike so this Shimano XT RD-M786-SGS or Deore RD-M6000-SGS would work fine"

I destroyed my RD today with a a not so elegant landing on the table top. twisted beyond repair, tweaked the chain too, and it will need replacement. I cannot find the shimano m5120 long cage anywhere online. My cassette has 46t, and neither of these listed seem to allow for 46t... I'm a bit new at all this, so don't completely understand the compatibility with the two mentioned. Not sure an upgrade is worth it for me, just looking to replace with similar. Thanks

UPDATE:
After doing some research, I found a SLX M7100 which seems to be very similar match, and a slight upgrade. Fit perfect, not too much calibration needed either... after re-viewing some YouTube videos got it installed. I bought a new chain too and had to shorten a few links without a chain break. (ended up grinding down pin on one side and using nail-set to punch through). I also had a shard of the dorkdisc stuck in between the cassette and spokes which would not allow the freehub to move, once that was removed everything works great. Back to the trails.