Park your car in an open space with enough room to maneuver on either sides of the car. Move the gear in the parking mode and apply parking brakes. Loosen the nuts on the rear wheels with a wrench or a spanner. Now use a jack to lift the rear side of the car till the wheels become an inch or so clear of the ground.
Though not necessary, but it is always safe to use back up stands in case the hydraulic system of the jack fails; to protect both you and the vehicle from any harm.
Remove the nuts from the tires and take out the rear wheel. You will see a closed metal container with two holes in it. Push the required bolts through those holes and the assembly will come out. Use WD-40 to remove rust and soften the brake assembly.
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Inspect the old motor and decide whether a change is actually required or not. If the old rotor is visibly thin and has a rough surface, then it’s time to replace the rotor. Unpack the new rotor and spray it with a brake cleaner. Also clean out the brake pads with the help of a sand paper.
Take out the old motor after unscrewing the bolts and replace it with the new one. Put the metal casing back on it and use a hammer to fully press it against the brake case.
Put the wheel back in its place and use the wrench to mildly tighten the nuts. Take the car off the jack and now tighten the nuts to satisfaction.