So if you're going to do just one thing to imrpove your office chair ergnomics, make it a height adjustment. It's as easy as standing up.
As you stand in front of your chair, adjust the height so the highest point of the seat (when in the horizontal position) is just below the knee cap. When you sit down, your feet should be flat on the floor, with your knees at a 90° angle, the ideal lower body position for sitting down at a desk.
Adjusting your chair height is also a great way to check up on the ergonomics of the rest of your workstation.
If you can't fit your legs under the desktop (you should have enough room to comfortably cross your legs at the knees) or there is not enough space to move them freely, your desk is too low for you. Put it on risers, or invest in a desk that's the proper height.
If you can sit comfortably but you have to raise your arms to reach the keyboard or work surface, your workstation is too high. Find a way to lower your worksurface (a keyboard tray might help). Alternatively, you can adjust the chair height so your elbows are the same height as your desk and use a footrest high enough so you can lay your feet flat.
(Images: Shutterstock, Taryn Fiol)