If you make a mistake assigning files into cards, you can undo it 2 ways

From the assign screen

1) Go to the Assigned tab, and

2) Click the Rollback Latest Assignment icon. Note that if the document has already undergone additional workflow, (submittal, routing etc), this icon will NOT appear.

These actions will put the document BACK into the Unassigned tab, where you can again drag it onto the card list.

From the history screen (for the card)

1) You can click the rollback function

2) You can reassign the file to a new card. this is the only option if there has been actions against the card since the last assignment.