To replace a brake rotor, remove the wheel, unbolt and remove the caliper, remove and replace the rotor, compress the caliper, and put the assembly back together. If a brake rotor is bad, the brake pads on the corresponding caliper typically need to be replaced as well.
Pump the brake pedal about 20 times to relieve pressure. Loosen the lug nuts on the wheel to be serviced, and jack up the vehicle. Place jack stands under the appropriate points, and then lower the vehicle onto the stands. Remove the lug nuts and the wheel for access to the brake assembly.
Unbolt the caliper fasteners, and remove them. This may require a special tool. Remove the caliper, but keep it supported by hanging it with a wire from the vehicle frame. The weight of the caliper can damage the brake lines if allowed to hang without support.
Lift the old caliper off the mounting studs, and replace it with the new one. Remove the old brake pads from the caliper, and insert new pads. Compress the pistons on the caliper with a caliper spreading tool so it fits over the new rotor with the new brake pads installed. If the pads rub against the rotor when you are trying to put the caliper back in place, it is not fully compressed.
Reattach the brake caliper fasteners, and torque the bolts according to the caliper specifications. Put the wheel back on, and lower the vehicle to the ground. Check the brake fluid, and add some as necessary.