|1. unplug power cord|
2. lay tower on right side
3. remove left side panel - probably have to remove 2 screws in the back that hold it in place
4. locate the CMOS battery on the motherboard
5. press metal tang to release battery. Here's a picture:
6. check model number on old battery, buy same model number. They're only a few bucks & can be purchased just about anywhere that sells batteries
7. install new battery, reinstall side panel, stand tower upright, plug in power cord
8. start system & immediately enter the BIOS (CMOS setup) to correct the date/time & other settings.