Use a screwdriver to remove the pair of long screws that connect the old telephone jack to the wall. Pull the telephone jack gently away from the wall without pulling out any of the wires.
Record the location and color of the wires that are connected to the old phone jack. The telephone wires that go into the wall should be connected to terminals on the back plate of the phone jack. These terminals in turn are connected to pins in the phone jack itself.
Identify the positive (tip) and negative (ring) telephone wires if the old telephone jack isn't connected. The tip wire is usually green, and the ring wire is usually red. You may need to experiment with different wires in the unlikely event that your telephone wiring uses nonstandard colors.
Loosen the terminal screws with the screwdriver and remove the wires from the terminals. You may need to cut the wires and strip off some insulation if the wires are too badly worn to provide a good connection. Wrap the tip wire around the terminal that's connected to the left innermost pin. Wrap the ring wire around the terminal that's connected to the right innermost pin.
Tighten the terminal screws, and plug a telephone into the telephone jack. Test the telephone jack by listening for a dial tone on the telephone. You can screw the telephone jack back into the wall once the telephone jack is properly connected.