When a transaction has a green check mark next to it, it means that transaction has been finalized and cannot be edited or changed.

To make changes to a finalized transaction, 1) simply click into the transaction and 2) in the upper right hand corner click on the green checkmark.

You will see the color of the check mark has changed from green to blue and the transaction can now be edited.

You will the able to update the categorization as shown below.