Turn off the Eureka Boss power switch and unplug the power cord from the wall. Lower the handle to the floor.
Flip the power head of the vacuum over so that the brush is facing you. Remove the three screws securing the bottom cover of the power head with a Phillips-head screwdriver, just as you would when replacing the belt.
Remove the broken belt from the unit, and lift the brush roll away from the unit. Inspect the side slot where the ends of the brush roll insert into the power head. Remove debris from the side slots. Sometimes a carpet strand or large amounts of hair will build up in the bottom of the slot, preventing the brush from rotating.
Clean the brush roll bristles, removing carpet strands and hair. The bristles on the brush should be free of debris.
Install the new belt around the brush roll as you would when normally replacing the belt. Reinsert the brush roll into the power head, ensuring that the end tabs fit snugly into the side slots. Install the bottom cover.
Flip the Eureka back into its working position. Ensure that when you vacuum over edges of rugs you raise the power head using the height adjuster. This prevents sucking the edges of rugs and carpets into the brush roll which can also cause belts to break.