how to change the rear brake pas on a 1985 honda 200x? thanks

Single disc in the back? Should just be two bolts that hold the caliper on. Remove. Slide off the caliper. Pop of the pads. Then try to compress the piston all the way back into the caliper. It's pretty much impossible to put new pads on without squeezing the piston back. C-clamp or some good old fashioned elbow grease. Just put a block of wood that the caliper can sit on, then something like a ratchet extension with a socket a little smaller than the piston. Place it on the piston in the caliper sitting flat on the wood and PUSH!!!