Clear HU object assignment key

I have an issue where the HU assignment key on the HU status detailed view shows that the HU is tied to a specific delivery. The delivery has already been PGI'd, and the HU is not on the delivery. I need to know how to clear the HU object assignment so that I can pick this for a delivery and PGI it. I currently get an error that its already assigned to a delivery (when it's not).