There are 40 different variations on the heading flymo 330! so a model number might help but....
The belt change is quite straight forward.
UNPLUG THE MOWER
You need to remove the blade, any spacers and fan from underneath. Next remove the motor cover (in front of the grass box)
Carefully remove the wires from the motor (You may have enough slack to leave them connected, depends on the model.)
The motor drive plate is secured from underneath with 4/5/6 screws. Undo these to release the motor and drive plate. Check for damage, remove the remains of the old belt.
Fit the new belt around the motor pulley and then stretch around the blade drive pulley - it will be tight and you may have to release the motor mounting screws to get it on.
Reassemble in reverse order and get mowing :)
Admin - www.gardenmachinerypro.co.uk, May 2012