If the voltage is below about 2.75 volts, the battery will have to be removed. Using a soldering iron, melt the solder holding one of the solder tabs to the board. While the solder is molten, pull the solder tab away from the board with a pair of tweezers.

Repeat the procedure for the other solder tab.

The battery is held to the board with a piece of double sided fabric tape. This will probably rip apart when you pull the battery off.

It would be a good idea to put shrink tubing around the new battery to ensure it doesn't short out to anything on the control board.