Unplug the machine and turn it on its side.
Remove the soleplate by twisting the screws into the unlocked position. All Dyson vacuums have screws that you can twist by inserting a coin into the slot. Remove the soleplate to expose the brush bar and belt.
Remove the belt if it is broken by pulling it out of the machine.
Install the new belt by wrapping one end around the notch on the brush bar and the other end around the black motor shaft. Check the belt is not twisted.
Push the brush bar into the brush bar housing. The belt will be very tight.
Remove lint, dirt, hair, strings and other debris from around the brush bar and belt if the belt is not broken. These things can clog the brush bar and cause the belt to break.
Replace the soleplate and tighten the screws with a coin.