Open the dishwasher door, and remove the lower rack.
Lift the tabs on the underside of the wash arm, and then lift the wash arm off of the lower wash-arm support.
Unscrew the plastic cap in the center of the top of the wash arm, if there are no tabs below the arm. Lift the wash arm and filter guard out of the dishwasher.
Remove the wash arm from the filter guard be releasing the tabs securing it in place, if applicable to your model.
Look on the underside of the wash arm for two metal heat shields, located near the end of each arm. If the shields are present on your dishwasher's wash arm, use a flat-head screwdriver to depress the tabs securing them to the underside of the wash arm, and transfer them to the new wash arm.
Snap the new wash arm into the top of the filter guard, if applicable to your model.
Place the new wash arm onto the wash arm support, and then press down to engage the tabs, or replace the plastic cap in the center of the arm, threading it clockwise to secure the arm.
Replace the lower dish rack, and then close the dishwasher door.