has some power applied to the mobo anytime that the power supply is plugged into a 120 volt outlet.

If the supply has a switch on the rear turn it OFF [0 position]. If not, unplug the supply. Turning OFF is best because then your system is still grounded to the house wiring via the third wire of the AC Power plug.

After removing the cover simply touch any of the metal of the case to discharge any static charge. Touch the metal frequently. Dry winter air allows build up of static charge easily if you are moving your feet on a plush carpet.

Remove the battery with care to insure that you don't bend the positive contact such that it doesn't make a good connection to the new battery. Easy to bend it and lose the tension.

Don't pet any cats while working inside the computer LOL.