how does one collaspe the piston on the rear brake caliper?

if i am not mistaken, the piston just pushes in. if the caliper has a cable attached to the back of the caliper itself, then the piston screws into the caliper. you can rent a tool from a parts place to do this operation, if necessary. Roy

how do i change rear brake caliper

You have to either buy or rent a Disc Brake Piston Tool. I bought one from NAPA, it's a Napa Service Tool #3163. On my 2005 the rear caliper piston screws in but you need that tool. Joey

The best and cheapest way at "NO COST" is to use a pair of needle nose pliers and turn the pistons until they retract..... Easy