Remove the wheel. If its disc brakes remove the brake pads and caliper. Remove the big nut with the cotter pin from the spindle. Bump the hub and remove it. The outer bearing should come right out, the inner bearing may have to be tapped out. Make sure not to damage the races and the rotor surface. Pack and replace the bearings and the rotor. Tighten the big nut enough to get the slack out where it spins freely but doesn't wiggle any. Replace the pads and caliper and then the wheel.