Turn off the electrical power to your Frigidaire dishwasher from your kitchen circuit breaker.
Open the dishwasher door, then pull out the bottom and top racks all the way to remove them from the dishwasher.
Grab the retaining nut in the center of the middle spray arm, and turn it clockwise.
Lift up and remove the middle spray arm from the inside of the dishwasher.
Insert the new middle spray arm onto the shaft, then tighten the retaining nut back into the center of the spray arm.
Close the dishwasher door, and turn the electrical supply back on from the circuit breaker.