How do I replace the memory battery?

klkokkonen, I can’t say this with 100% certainty, because I can’t locate the service manual off hand, but many of Nikons cameras don’t use a typical CMOS battery (eg CR2032 like computers etc). These cameras use on board Semb electrolytic capacitors, in place of batteries. These are designed to keep time and date for 4-6 months (in deal circumstances).

In order to replace those (as electrolytic capacitors are susceptible to major increase in ESR) you’ll need to open the camera and replace the caps. That would best be done with schematics and board views!