I have two Dell OptiPlex 9010 SFF computers, and one of them have been unused and unplugged for a long time. So long in fact that the CMOS battery is now dead. But replacing it was simple.
The battery is a CR2032 and is located right next to the first PCI express port, if you have a card there you will need to remove it to get access.
A simple push on the handle and the battery popped out.
I turned the computer off, and unplugged it for a few minutes… No warnings when powering it back up, success! 🥳