There's a bolt on the back of the caliper. Remove that and you can stick an Allen key into the bolt hole to rotate a mechanism that retracts the piston. The gear mechanism can fall out so don't lose it. Don't forget to readjust when you put in new pads. Readjusting the piston means to rotate it until the pads touch the rotor, and then back the pads off the rotor by 2/3 of a turn.
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