Veronica,
Sometimes it can be hard but make sure you have the brake fluid chamber cap off. that is where you put the brake fluid in.
leave the old pads on and take 2 tire irons or large screwdrivers and pull them in opposite directions .. the caliber will start moving back into the boot or if possible get a large C clamp found at any hardware store and use that to compress the clyinder back .. make sure your cap is off on the filler chamber.. if not you would need to lossen the brake bleeder valve and then possibly have to bleed the sytem ..
hope this helps ..
James, May 2005