Solder the replacement battery back in place. Note which end has the positive (closest to the shaving head), usually this can be identified by a small ridge/dip on the battery case (if not clearly marked)

Replacement battery must be of the same voltage, although the capacity my be larger if needed.

You may need to trim (using scissors) the solder tags on the battery to make them narrow enough to fit through the holes in the PCB / circuit board