Look for your pre-motor filter. Depending on the model, this may be located in a tray beneath the debris canister or at the bottom of the vacuum bag compartment.
Slide the tray out, and remove the filter. For Bissell vacuums with bag compartments, pull the bag door outward from the top of the vacuum, and set the door aside. Slide the grille out at the bottom of the bag compartment, and remove the filter.
Wash the filter of dirt with cold water and a mild detergent. Continue to clean until clear water runs from the filter.
Wring excess water from the filter. Allow the air filter to air dry completely before attempting to reinstall.
Insert the filter back into the filter tray, and push the tray in to close. For models with bag compartments, reinstall the filter, followed by the grille. Replace the door, and snap it back in place.