don’t sit at that chair.
reposition it to fit you.
move around and stretch your back
Adjust the height of the chair to set it up so your feet can both be place flatly on the floor. and pull your shoulders back so you sit up straighter in the chair. also possable place a towel in the middle of your back to reinforce the natural curve of your spine
Get a good back support for your chair. Here’s what I use:
This should do the trick. Otherwise maybe get another chair. This sitback rest or the slimback rest (the sitback rest is a little thicker) does the trick for me. I did have a old, cheap, bad computer chair once though that I just got rid of because I think it was such a poor chair overall and it was on it’s way out. But if it seems to be a decent chair, just use one of these lumbar supports and you should do great.