The rear brake pads are fairly easy to remove and replace, but you will need the special tool to compress the piston back into place. You will need a 13 and 15mm wrench, remove the caliper by removing the two 13mm bolts, you will need the 15 to hold the back from turning. Once the caliper is off, see picture below to reinstall the piston back into the caliper.
Harbor Freight has them in stock. You can also purchase on line for around $19.00 - $59.00
click on the link for the tool that is $19.00