i have a renault scenic 1.4 how do you compress the brake piston to fit new brake pads on the rear end

Although l do not own a renault l have been a mechanic for over 15 years.
Almost every model of auto that l have ever worked on l have used a large c-clamp to compress the piston after removing the cap from the master cylinder.
Hope this helps

If what ''Starnut68'' says does not work maybe you have a seized piston. (Bit of advice in case you've already tried!)

on the rear brakes of some vehicles the piston gos back by pushing and turning at the same time, there is a tool to do this job
the piston will have four cut outs around the edge if yours is the push and turn type, if not then the piston is most likely seized in the caliper

Rear brakes will have a handbrake mechanism in the calliper which often works by rotating the piston against a threaded part. You will have to rotate the piston while pressing it in. It is a fiddly job but it can be done. Much easier to buy the tool (Halfords?) Whatever you do use something that fits the piston end so you do not damage the face, and DON'T use force. If you try to force it in you will break the calliper!

As its the rear one, look for grooves in the piston end, you have to inset a flat bar (a big flat blade screwdriver is ok) and rotate the piston anti clickwise back into the caliper. Dont try and jam it back with a G clamp (which is fine for the fronts by the way) or you will destroy the threads.
The handbrake works by rotating the piston clockwise.