@Normsthename It is hard to describe as I have only ever seen it done once and not by me. @Lightly did mine. Take the rear wheel off and look at the caliper there is a slot in the side about 1.5 to 2" long and .25" wide into which you stick a flat bladed screwdriver to use as a lever. then it is a matter of unscrewing the piston by rotating the screwdriver upwards.(on one side and on the other wheel downwards) to make the piston stick out further before the handbrake is applied.
Before you start make sure there is slack in the cable. Wind the pistons out and then adjust the cable. What Mark did with mine was reset the handbrake cable back to the original (new) position and then adjust the pistons to the correct position (so the brakes are not locke don!) and then make any minor adjustments to the cable.