No easy way... You may need to pull out the blower motor and get in there with a vacuum cleaner. You could have a look if you can see anything via the pollen filter holder (removed first).
If the bits are downwind of the heater matrix and are already in the ducts, you might be able to blow them back down into the foot vents and then out with the blower motor. Careful use of the air direction control will help Try a cold hair dryer first. Then compressed air if no luck.