Some upgrades require you to use leverage to remove the board as the upgrade itself will prevent it from being removed the normal way. Slide it as far as it will go to make sure the front edge of the board is clear of the chassis. Next, insert a spudger or (if you're careful) a flathead screwdriver in the gap between the board and chassis rail. Apply gentle outward pressure so you flex the chassis rail outward while pushing the motherboard upward and out of the chassis.
The chassis rail only has to move outward about a millimeter, and it'll spring back to its original position once you get the board out.
Do the same to get the board back in. Place one edge of the board all the way into one rail, then place the spudger between the board and other rail. Gently pull your spudger inward to flex the rail outward and move the motherboard downward back in place. Just make sure the front edge of the board is clear of the lip of the chassis before doing so.
I did it so many times at one point that I was able to do it by hand. The chassis rail really doesn't need to move outward that much.