Are you talking about the back of the case to get to the battery? If so, turn the phone over and upside down. On the bottom of the phone (which is now up) there is a little tiny slit in the middle where you can stick your fingernail to pry it open.
If you are talking about the $20 Sprint case, turn the phone over, right side up. Put your thumb on the middle of the back, while prying with your fingernail between the shell at the top (where you put the charger). Pop one side, then the other and it comes right off.