Can't link you to the exact steps as you didn't post what version you have.

But the battery is a standard CR2032 coin cell battery.  Available most stores as that battery is used in many devices.

Its located on the motherboard.  

To replace, unplug the AC power cord from the power supply.  Press power button for about 5 seconds to drain any residual power.  Open the case and locate the battery on the motherboard.  Remove old and install new.  Reconnect AC power cord.  Power on.  The BIOS will be set to factory default.  No special tests will run, just standard POST (Power On Self Test) that runs every time its powered on..  If you had any custom settings in the BIOS you will have to reset them, otherwise no further action needed.

Hello fireberd and Thank You!

I'm using a FULL Sized 790 Mini Tower (Full Sized PCI Slots) and running Windows 7 Pro - SP1 - i3-2120 - 3.30 Ghz - 16 GB RAM

I believe I changed a Bios setting to adjust for an internal Storage Drive and to run an internal esata dual port card. Will I loose those settings?

Thank for the instructions above!

The BIOS will be reset so note any changes so you can re enter those after the replacement.