2006 uplander-how to push the rear brake cilinder back to change the pads ?

On those brake calipers you need to turn the piston as you push the piston in. Of course, there is a tool for doing this. Using a C-clamp or a caliper piston tool, push the piston in, then with the other brake tool, turn the piston clockwise a turn or two, and then repeat with the C-clamp. Spraying the piston and seal with WD-40 will help in turning the piston to collapse it back into the caliper.