how do i replace the battery on my 99 c5

No different than any other auto. Its in the back corner of the passenger side engine compartment. First unhook the battery terminals. Negative first then positive, this minimizes the possibility of more sparks for accidental grounding and causing shorts. Probably side of battery bolt that screws into the side through the cable, hard to get at. If you have a flat wrench that has a built in ratchet that will work best. Try your best to make the first disconnect clean from the battery, I always seem to have a problem with this, I think I'll get a switch to put on the cable so I can turn the power off. Then there is a hold down clamp on one side of the battery that you'll have to remove with a ratchet and extension because it down beside the battery and again very little room, this will loosen the battery case so you can lift out. Because it's a small area and the battery is heavy you might want to get help to lift, and work the battery loose, and remove the battery, be careful of mashed fingers. Be sure you get a new fully charged battery with the right cranking amps. from your parts store, they should be able to tell you specifically which one with what ever warranty you choose. Place it back in reverse order of the removal. But this time you connect the positive terminal first. Then your done. There are battery replacement videos on UTube.