Usually, the belt that runs the brush roll in the Hoover WindTunnel stretches out after about every 6 months to a year and you'd need to replace it often. (You should know that despite this being a general instruction, each vacuum type is different from the other one.) To replace a Hoover WindTunnel belt, follow the following instructions:
Unplug the vacuum cleaner and turn it upside down to reveal the vase plate
Unhook the base plate by unscrewing or lifting the red tabs.
Lift the base plate to reveal the belt
Remove the brush roll and unhook the belt from the attaching motor shaft pin.
Replace the brush roller and the new pin.
Return the plate in place and screw it up.