My current setup is 56/39 chainrings and 9 speed 12-23 cassette. I having a hard time climbing with this setup. Will changing the cassette to 11-28 enough or just go with 11-30? Will I need to change the hub and the chain too?
Any help will be highly appreciated.
Thanks

You will have much larger gaps between gears. Assuming you are not a TdF rider, you are better off with smaller chainrings. You need to know BCD to see what fits.

Bicycle Gear Calculator

An 11-28t cassette will probably get you where you want to be. I was running a 12-25t, and pushing to a 28t made a nice difference.

^^^^^^
this

What bike do you have and roughly where do ride?
Ah, just looked at your other posts. You have a pretty roadcentric tight cluster which may be good for racing on moderate terrain but lacking in range for serious hills. Location makes a difference as gearing for Florida is somewhat different than Colorado.

Around here (Western Canada) we have hills, and mountain passes, and I ride a Shimano Tiagra 50/34 with an 11-34 cassette (4700). On long sustained climbs I couldn't imagine using your gearing.

I’m currently running a 50/39 crank and a 10sp 12-23 cassette. I love the close ratio cassette, but I’d like to extend the low end a bit. I’m looking for a compact crank (50/34) to get me some lower gearing without messing with my close ratios. Rolling terrain without any serious hills. I have a 13/29 cassette for climbing routes

Must be nice to have good knees, lol. I run 50/34 with 11-34 cassette on two of my bikes and the CAAD12 came with 52/36 pushing an 11-30.. that's as tall as I like nowadays.

If you switch out your cassette, you might want to also switch out the chain as well, depending on how much wear it has.
Are you sure your big chainring is a 56 tooth? I have never heard of a chainring that big.
I don't doubt you are having trouble climbing hills with your current set up. Your lowest gear of 39 - 23 is a fairly big 44.7 gear inches.
Switching to a 28, will help a little, as you will get to 37 gear inches but will it be enough? That IDK. I can only speak for myself in saying it wouldn't be low enough for me. For the riding I do and my level of fitness, I look for a low gear of 25 or 26 gear inches. On a 39 tooth small chainring paired with a 34 tooth cog gives you 30 gear inches, and 36 tooth, 28 gear inches. The problem of course is, you will have fairly huge jumps between gears, and it might not even work with your back derailleur.
You might want to switch to a Compact double set of chainrings 50/34. AND switch out your cassette to one with 32 tooth biggest cog. Then you are down to a very usable 28 gear inches.