You need to remove the hubs, then remove the spindle nuts and lock rings. Only after these are out can you get the rotor assembly to come off the steering knuckle. You can then drive the wheel studs out of the hub and rotor assembly and replace the rotor. And as all the really good service manuals say, "Install is the reverse of removal". Just take care not to over or under torque the spindle nut/lock rings lest you end up with terrible wheel wobble (Manifests itself initially as terrible negative camber after installation) or so much pressure on the wheel bearings that you burn them up.