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.

This drain bolt is so tight not sure i can remove it. I even used a bar on top of the ratchet, rounded some of the bolt. Only thing that fits is a 15 inch socket. I sucked some of the fluid via the dipstick but only 3 quarts maybe came out. Nothing leaking around the pan

Just removed the pan completely. It is still sitting on top of the exhaust, there maybe just a bit more fluid but i got out about 2 3/4 liters. May have been leaking from the front over time. Does this usually come off without the exhaust coming down?

With a factory exhaust you can usually get them out, some of the aftermarket ones don't hang correctly and block the pan. From the description of the drain plug I'm betting it hasn't been changed by dropping the pan, but it might have been changed with a pump. I've done that on multiple vehicles myself. Pump through new fluid and then drop the pan and change the filter, that gets more new fluid into the system, but you use more fluid.

How much fluid should i start with after replacing filter and gasket. And with these cars do i check the level with engine off

For a spill and fill you should need 6 quarts. For the fluid check you should run it a bit to get the fluid warm, You can access a transmission fluid temp reading through the drivers display, using the trip button to toggle the display to transmission temp. Then check that the fluid is in the hot area with the engine running at idle and transmission in park.