how to change out NVRAM battery in an imac

You take it to your local Apple Authorized Service Provider to have it done so you don't damage your iMac. That would be the wise thing to do. Why do you feel the NVRAM battery needs to be changed, if the computer has been in use it is extremely rare to change the battery.

I agree with the others. I've not needed to change an internal backup battery in a Mac since I retired my 2003 PowerMc G4 in favor of an Intel iMac. That makes the "why" question that rkaufmann87 poses super-important.

Have you tried resetting the NVRAM first?

Reset NVRAM or PRAM on your Mac - Apple Support