Assuming you are referring to the tailgate on a DSLR camera, there are a few ways to remove it. The first is to simply unscrew the four screws that hold it in place. This will require a Phillips head screwdriver. Be careful not to lose the screws, as they are small and easily misplaced.
Another way to remove the tailgate is by gently prying it off with a flathead screwdriver or other thin implement. Start at one of the corners and work your way around until the tailgate pops off. Again, be careful not to damage the camera body in the process.
Once the tailgate is removed, you'll have access to the camera's memory card and battery compartment. Simply replace the battery and/or memory card as needed, then reattach the tailgate using either method described above.