To fit a new rear brakedisc yoy first have to do some removals.
First remove the exhaust:

Remove the upper bolt from the passenger footstep.
Then also loosen the bolts on the exhaust downpipe below the engine.

When the exhaust is removed. Use a flat screwdriver to remove the black plastic cover in the rearwheel:

The use the same screwdriver to remove the circlip

Then use the black 55mm hexagon on the rear bolt

Use a good socketwrench (24mm to fit the wrench) on the rearwheel bolt.
One tip. It is better to do this with 2 people. Put the bike in first gear and the second person pushes the rear brakelever down. Then the wheel is going nowhere.
When the bolt is off, get the rings of and pull gently on the rear wheel. If everything went well this is what you should see

The use a correct hexagon wrench to remove the bolts from the rear disc, then drop down the disc and remove it. If this doesn't work, remove the bolts of the rear brake caliper en pull this upward.
Installation of the new disc is in reverse order.