I have just bought the same model today and enclose the answer from the instruction book.
How to remove the Bottom Plate;
Unplug the vacuum.
1. Place vacuum in the low position by pressing the handle release pedal twice. Then, turn the vacuum over, exposing the underside. 2. Remove the four screws, holding the lower plate in place. 3. Now remove the bottom plate.
How to replace the Belt
1. turn the unit over and remove the bottom plate as described above.
2. Remove the agitator holder. Take out the agitator assembly and the motordrive shaft.
3. Loop the new belt around the agitator pulley and the motordrive shaft.
4. Reinsert the agitator assembly back into housing. Turn the agitator assembly by hand, making sure the belt is not twisted and all rotating parts move smoothly.
5. Reinsert the agitator holder. Be sure that the agitator holder is in place before fastening the bottom plate.
6. Hook the front end of the bottom plate to the two ribs at the front end of the housing. Press the bottom plate into place and fasten it by four screws.