step one: open the two side clips on the diffuser. These are black plastic plugs located at polar opposites of the diffuser ( the corners) you wanna pull the locking pin out to release them.
step two: remove diffuser
step three: two holes hiding under the diffuser, those are for the main bolts that hold the bumper on, undo those bolts
step four: same clips that held the diffuser and some screws hold the mudguard to the bumper. jack up the car by the diff. Remove both rear wheels, undo those clips and bolts.
Step five: slide bumper off.
Step 6: redo in reverse.
You can also remove the bumper off the rebar if you are reusing the stock one ( m3 bumper FITS the stock rebar and styrofoam spacer)
Simply take off the mouldings, and pop open the lock clips, either by pulling the lock caps out, or if you have the pin type, shoving the pin in til it pops out the other side... you wanna keep those pins.
undo the mudguards and pull the bumper cover off the rebar.