Here is the procedure for replacing the rear brake pads on your Volvo:
Compress the caliper piston back into it's bore while rotating the piston in a clockwise direction. This does require a special tool to do this without damage to the caliper. Here is a link to a web site where you can see what this type of tool looks like, or purchase it if you wish:
Once the caliper piston is depressed back into the caliper bore, then it is time to reassemble in teh reverse order of disassembly. The brake pad with the sheet metal wear indicator clip goes on the inside towards teh caliper piston.