I got a short leg cast today and my knee is very stiff what are ways for the stiffness to go away?

Motion: Flex and extend your leg at the knee.

Move it: I assume you were taken out of a long leg cast and converted to a short leg cast. If so, the motion will gradually come. I would just try to heat it up prior to stretching with warm compresses and then stretch in both flexion (bend) and extension. It may take a couple weeks but it should improve. Your extension at this point is probably more important than the bend.