how hard is it to change the cassette? and are they universal? I have an older centurion and it only 10 gears, I would like to get a 7 or 9 speed cassette.

how would i know what works?

can i even do i t?

If you have 5 in back, then it's a freewheel, not a cassette. You can only go to 6 or 7 with a freewheel. You would need a remover tool to match your brand, and probably a new chain, especially if you go with narrow ultra-7.

To go to a cassette with 7-10 gears, you'd need a different wheel with a cassette hub, then a matching chain.

Pretty much nothing is universal ... things have changed over the years, but some things are more common or standard than others (700 c tires, English threaded bb) .....

This will tell you 99% of everything there is to know about bikes:

https://www.sheldonbrown.com/

No, they are not universal. you will need to know what you have and the proper tools to remove and reinstall it. It's not dificult at all if you have the tools.
Go over to YouTube and look up cassette replacement. That will show you more that we could say in words.

thanks for the info , very helpful

And if your bike has indexed shifting, you will usually need to change your shifters to match your new cassette or freewheel. The benefit is usually not worth the price.

It does have friction shifting, so I would not need new shifters. it looks like i can get a 7 speed shimano freewheeel to replace mine online. with tool and new chain $40 maybe it is worth it.

If yours is 5 speed, then the axle may not be long enough for a 7 speed freewheel. If so, you'll need a longer axle, or move a spacer from the left side to the right to shift some length to the right side.