My Mk1 has the 'Clipper' factory installed body kit, which includes "modernised" body-coloured bumpers (as opposed to the chrome bumpers of older Golfs). I had to remove the rear bumper to do some repairs on it.
See also How to remove front bumper.
You'll need
The corners of the bumper are screwed and riveted to the plastic inner wheel arch. You have to remove the wheels to get at them. Use jackstands to be safe.
With a metal drill bit, drill out the old rivets (choose a drill bit that is slightly bigger than the hole in the centre of the rivet cap. Drill until this cap becomes loose. Pop the rivet out backwards.). There are about 5 rivets and 2 conventional screws each side.
The interior panels to the left and right of the inside of the boot/trunk cover the main bolts that attach the bumper. Remove these with a screwdriver and trim removal tools. Be careful as they're just plastic and can break.
With the interior panels removed you should be able to see 2 large bolts either side. Remove these with your socket. Don't worry too much about the bumper slipping out as it has quite long 'arms' that extend into the car.
You can now slide the bumper out. Be careful! It's heavier than it looks.
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