If you need to move an influencer from one Influencer Hub Assignment to another, there's an easy way to do it:

Open up the Campaign, and expand the Assignment:

Hover over the influencer that you want to move. The 'three dots' icon on the right side will appear. Hover over that to bring up the dropdown menu:

Select 'Move to another assignment', then use the dropdown menu to select the new Assignment you want to move this influencer to: