remove the grass box, turn the unit upside down, making sure you can tilt it to put a hand into the area vacated by the grass box.
remove the blade and fan.
now you will see 4 shinny tork head (star) screws, there are a further four in recessed holes in the casing, support the unit by putting your hand into the grass box area before removing the last screws. now you will be able to see both pulley wheels, place the belt on the motor pulley and on one side of the blade drive pulley, rotate the pulley to 'pull' the belt on to it - reverse the previous directions - job done
LD, June 2008