I accidently dropped off a parcel that was meant to be returned via UPS. How may I retrieve it?

Go pick it up. Because they can't scan it in. This is why you always wait to get scan from the employee versus leaving packages there without a scan in

If you dropped it off with a carrier or at your post office you may be able to retrieve the parcel at the carrier's post office or the post office you had dropped it off at. If the parcel moves on to a processing center, the postal workers will route that package to UPS, though it may cause a slight if any delay.