My mistake... i meant the wheel bearings in the rotor that ride on the steering knuckle, they could use repacking or need replacement, depending on condition.
Best to wipe away all the wheel bearing grease, check the condition of the bearings and race, look for any loosness, excessive wear or pitting, if everything looks satifactory to you, repack the bearings thoroughly with high temp bearing grease. Lightly tighten the castlenut till theres a slight drag on the rotor and theres zero back and forth play, then line up the hole in the steering knuckle and castlenut, use a cotter pin and bend the open ends around the nut itself to lock the nut into place.