Isn't there a drain plug on the pan? There should be a drain plug and
fill plug.  I do not believe there is a dip-stick on this GM auto
transmission, which is supposedly "sealed, maintenance free" .

The best way to service it is to have a "pro" do it with a power
flush... they remove the pan, drain and refill the torque converter
with a pump.  If you find a shop you trust (with real mechanics and
not high-school kids who work on your truck the same way they bashed
on their Tonka trucks a couple years before) and is not too expensive
this works out well.

I generally drain whatever fluid I can... around 1/2 total on my
Maxima... and refill. Every 15-20K miles.    AT fluid doesn't
degenerate in the same way engine oil does so running with a mix of
old and new isn't an issue so long as you do it more frequently to
keep it fresh.