Carefully flip the board over and note the two solder joints connected to the battery cables.

De-solder the two solder joints and replace the battery. You will have to re-solder the battery to these contact points.

Alternatively, cut the wires one-by-one before you desolder them. This will help prevent unintentional short circuitry.

Be very careful not to short the battery wires. Doing so may overheat and ignite the battery.

Be sure to maintain the correct battery polarity when you re-solder the wires.

The back of the board is shown for reference.