I don't think this model has an adjustable motor :(. Its a stretch fit belt.
Unplug it, Take the top off, turn round and remove the panel near the base, this will give access to the motor pulley, Feed the drive band down from the top over the motor pulley. pull up and place the belt on the drum pulley. with both hands hold the belt to the pulley whilst turning - its tough but will go on as you turn.
Spiny, November 2008