Changing transmission fluid?

Hummer H2 is basically a Chevy Tahoe in a body kit, Uses the same 4L60E as the rest of the GM line. 5 quarts for a typical spill and fill. 11 quarts if you pull it apart and dry it out. Uses Dexron 3 fluid and 24208576 filter. Replacement is typical. get it up in the air, if it has a drain plug (some did, most didn't) or remove the pan bolts and try not to take a shower in fluid. Drain it, wipe off and inspect the magnet for chunks and excessive fuzz. If it looks good, swap the filter and the seal that is inside the filter port, install the new filter seal by tapping it in with a hammer and driver. Make sure the pan flange is actually flat, I've seen a lot of them bent. Install the new gasket and tighten the pan bolts down to 97 INCH pounds in an alternating cross pattern.
Refill through the transmission dipstick. I like to also disconnect the cooler and blow it out to get a bit more fluid out. Run it through the gears and check the level.

Have not seen but unusual name brands for Dexron iii, what brand would you use?

I buy fluid in bulk through a local distributor and for those vehicles that spec Dexron 3 I use Delco Type 3H normally. However you can get Valvoline, Amelie and Motul Type three online if you don't have a local source.