How do you change an air bag on a tractor trailer?

Suspension air bag or cab air bag? Typically, there'll be a quick disconnect for the air line to the air bag. You undo that (bleed out your air tanks before you do this). On one end of the air bag, there'll be a nut which needs to be unbolted. Once that's done, you unscrew the air bag counterclockwise and it'll come out. If it's a suspension air bag, you'll need to jack up the frame. If it's a cab air bag, you'll need to jack up the cab.

Anything more specific than that would require a make and model of the power unit.