For rear brake pads, how do you compress the caliper enough to get the pads out?

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The rotor wears just enough to create a small lip at the outer edge. How do you compress the caliper enough to get the pads out? I know you have to turn the piston to compress it but it's a catch 22 since you can't get to the piston until the pads are out and I can't get the pads out past the lip on the rotor.