Go slowly. Resist the temptation to muscle her. First just ask her to pick it up, don't hold onto it. You can even start by using a rope, a soft lead rope, just loop it around her pastern and ask her to shift the foot, release with praise when she does. You can progress to picking it up using the rope, just hold it up for a second, really close to the ground not high up, put it down before she tries to take it away. That's always the key - she has to learn that she can always have it back, that you'll GIVE it back, she doesn't have to take it. Hind feet are a sensitive topic for horses, it's where predators grab. So you have to be consistent and gentle but firm, without any aggression whatsoever, but persistent. I would work on it a little bit every single day, even a couple times a day if you have her at home, just very short sessions as Chandab said.
Is she kicking AT you, or just flailing her leg around trying to get her foot back? If she's flailing, just the suggestions above - your timing should be such that she never gets to the point of feeling she needs to flail (but if you do get there, then I totally agree with Servicemini, letting the foot go while she's flailing is basically rewarding her for flailing so try to wait until she softens and then let the foot go). If she's actually kicking AT you, you want to head this off now. I do this with a growl, if that doesn't produce a change, then send her away when she tries to kick, that's what Mama would do. Caveat is, if she's doing it from fear, then give her a bit more of a break.
