To change the belt...
First, disconnect the nozzle from the Kirby. Turn the belt lifter until it says "Belt is ON" at the bottom. Turn the nozzle upside down and pull on the front of the rug plate (side under the belt lifter). It is hinged, so it won't come off the whole way. Slightly bend one end of the rug plate bar and pull out the brush roll. Slip a new belt onto the brush roll and put it back in, making sure the screw heads on the brush roll ends are facing the bottom of the rug plate. Bend the attaching bar back into place, and pop the rug plate back into the nozzle. Turn the belt lifter counter-clockwise until it says "Belt is OFF" on the side, making sure the Magic Finger in the nozzle catches the belt. Place it back on the Kirby, turn the lifter back to ON, and you're ready to go!
The Kirby belt is the soft grey belt, part number 159056.