Unplug the Electrolux vacuum cleaner. Do not trust that a vacuum that is just switched off will be safe. Press the handle release lever located on the back of the cleaner's head, and lay the vacuum flat.
Turn the Electrolux over so that the bottom of the cleaning head is accessible. Remove the screws on the bottom plate with a Phillips-head screwdriver. Keep the screws handy for later replacement.
Turn the cleaner over, keeping the handle in the flat position. Remove the two screws above each of the wheels with the screwdriver.
Pry the upper cleaning head panel away from the body of the vacuum with a flat screwdriver or other pry tool.
Turn the vacuum back over so that the brush roll and the underside of the cleaning head are facing upward. Remove the two screws on either side of the brush roll with the Phillips-head screwdriver. Slide the brush roll from the slots at either side of the Electrolux cleaning head.
Slide the belt off of the brush roll. Inspect the brush roll for strands of fabric and other obstructions. Remove anything wrapped around the brush roll or stuck in the brush roll's bristles.
Remove the belt from the motor drive shaft.
Slide the new belt onto the brush roll and position it in the space previously occupied by the old belt. Place the other end of the belt over the motor drive shaft. Place the brush roll back into the two slots at either end of the Electrolux cleaning head. Replace the screws with the Phillips-head screwdriver to hold the brush roll in place.
Place the metal plate onto the bottom of the cleaning head and replace the screws with the screwdriver. Turn the vacuum over and replace the upper plastic cover. Screw in the two screws above the wheels. Return the handle to the upright position.
Plug the Electrolux in and turn it on. Listen for the sound of the brush roll making contact with the carpet. A low moaning sound will signify a properly spinning brush. Turn the vacuum over to ensure the brush is spinning if you cannot discern its motion from the sound.