Maybe it is obvious and I am missing it, but I cannot figure out a way to change these characteristics after importing a bunch of games.

How can I do this?

If you edit an item in your collection, there should be a set of radio buttons for Physical and Digital on the right side of the modal. If you select Physical, you should get a variety of fields to describe the physical condition of the item. In order to fill out the specific conditions, you would need to select either New or Used.

Thanks so much. Wasn't at all clear to me that I had to check New/Used in order to use those fields.