Once you have installed a new lithium battery and carefully clipped it into place with the bracket (make sure it is the right way around) use a pin or fixya micro tweezers to short the little contact in the hole marked AC with the case back that is metal.

This is one of those strange times when you NEED a conductive needle or tweezers to do it right. You will no it worked if the display resets to 12:00

The AC reset needs to be done or the module may doing a weird display. Even if it looks correct do the AC reset anyway just to be sure.