You can link an existing part to the collection or update the collection of the part that has already been linked with a collection.
If you need to edit a collection in the part, simply click 'Edit Part' on the far-right column in the line where the part exists and the edit window will appear.
From here you may edit the part number, and switch on the button if check the “Mark Parts as Universal Parts”, Collection drop-down visible. Select or update the collection and click 'Update Part'.
Your part will be updated with a new collection.
Please note that if your part already contains the collection, the previous collection fitments will be deleted from the marketplace for that part.