Once you get the calipers off and the pads out, you'll have to retract the pistons in order to allow you to mount the new pads around the rotors. It's easy to do. Just use the old pads and some channel locks and squeeze til the piston(s) is back on each side. This'll send your brake fluid up the lines and back into the master cylinder. Make sure you have a rag or something underneath so no fluid gets anywhere.