The Dell OptiPlex GX620 desktop computer comes in several types of cases ranging from a mini-tower computer to an ultra small form factor computer. All GX620 models use a small disk-type battery like those found in wristwatches to maintain system settings, time and date. While these batteries should last several years, they do wear out. A bad battery will cause incorrect time and date to be displayed and should be replaced. Replacing the battery is not too difficult, but care must be taken not to damage the motherboard.
Power off the computer. Remove the power cable from the rear of the case. Unplug any components that are attached to the computer, such as the monitor, printers, networking cable, etc.
Remove the left side cover. The GX 620 side cover has a latch at the rear of the cover. Push the latch inward and slide the cover back and away from the computer.
Locate the CMOS battery. It's silver, about the size of a nickel and is seated in a plastic socket. The battery is held in by a small metal tab. Use a pocket size slotted screwdriver to carefully push the tab away from the battery. Use a plastic toothpick to lift the battery out of the socket. Note which side of the battery is facing up.
Snap a new battery into the socket. Replace the side cover, plug in the components and the power cord and turn on the computer.