I just replaced mine, the flap had broken off leaving just a little bit of the "leash" in the body of the phone. I used a small knife to pry back the little bump guard near where the piece was stuck in there, then used a thumb-tack to sort of stab it and pull it out to where I could grab it with needle-nose pliers. I then gently pull it with the pliers while I pried slightly to give it room to come out.
If yours isn't broken off already I'd say just pull it lightly while prying the corner just a little bit.
Then I pried lightly while sticking the new one in, it popped in with ease since I had the corner of the bump guard pried open a little bit.