Bicycle wheel bearings should last for years and provide you with thousands of miles of travel—but they won’t last forever. In some cases they even fail completely if they haven’t been maintained correctly. Part of a maintenance schedule should include replacement of wheel bearings before they fail. The procedure isn’t that difficult if you have about an hour and a few small tools. Front and back wheels are similar in procedure to change bearings and only require slightly different tooling.
Remove the wheel from the bike. If you are doing the back wheel, remove the gear cassette freewheel first by turning it counterclockwise. If it's too tight, apply the freewheel puller. Hook it on, loosen the freewheel and lift it off.
Lay the wheel on its right side, and work on the left side of the wheel. Take a screwdriver and pry off the rubber seal. Slide a 15 mm cone wrench onto the cone flats on the bottom for the back wheel, a 13 mm wrench for the front. Now place the 17mm open-end wrench on the nut on top and loosen it by turning it counterclockwise against the cone wrench.
Stand up the wheel. Using both hands from both sides, unscrew the locknut, cone and spacers. Now slide out the axle from the right side. Using a small magnet, remove all the bearings from both sides and count them. Clean off all the old grease.
Liberally apply new grease to the inside of the hub where the bearings ride on both sides and to the cone that you just took out. Now carefully push the new ball bearings down into the grease one at a time---the front hub using 3/16-inch bearings, and the rear hub using 1/4-inch bearings. Now cover all the bearings with grease on both sides, letting the grease hold them in place. Grease the axle and slide it back in the hub.
Slide the left side washers, spacers and cone on first, then the right, and tighten everything finger tight. Lay the wheel back down on its right side, slide the cone-wrench on, then slide the 17 mm open-end wrench back on and tighten clockwise. Reinstall the rubber seal, and put the wheel back on the bike.