How do you remove an assigned seat?

I have assigned a seat to a passenger - how do I remove it and reduce the charges?

Step-by-step guide

Click the Select Seat button.
Select the seat that was selected previously (In the example below, I clicked 7C, which turned it red to show it was deselected) and click Save. This will remove the seat assignment.
Click the Charges button.
Select the "Seat Assignment" charge and click Edit.
Change the amount to $0 and hit Save.