How to remove and replace dishwasher spray arms:

Removing and replacing a spray arm
Note:
Disconnect the power source to your dishwasher before you conduct this or any other repair. Either unplug the unit from the wall outlet, remove the appropriate fuse from the fuse box, or flip the appropriate breaker in the circuit breaker panel.

Water is forced through the spray arms of a dishwasher by the unit's pump and motor assembly. The force of the water propels the spray arms, spraying water and detergent onto the dishes on the dishrack. Depending on your model, you may have only one spray arm located on the bottom of the dishwasher, or you may also have an upper spray arm located at the top of the unit above the upper dishrack. There may or may not also be a plastic tower extending from the center of the lower spray arm, whose purpose is to spray an additional blast of water at the dishes.

Remove the lower dishrack from the unit. If your dishwasher has a spray tower in the center of the spray arm, remove it now by unscrewing it with your hand. You should see a bolt in the center of the spray arm. Remove the bolt and then remove the lower spray arm by pulling up on it. If your particular spray arm is not held in place with a bolt, lift up on the spray arm to remove it from the unit. You may need to rock it back and forth a little to help ease the process.

If there are any washers used with your spray arm, make sure you return them in the same order they came off.

If you are replacing the spray arm, simply discard the spray arm you have just removed, and insert a new one in its place. To clean your spray arm, soak it in warm, soapy water. Rinse it clean with a high pressure spray of water. Return spray arm to dishwasher.