To Add/Remove a Tag to a contact go to your Active Campaign Action. From there, click the green "Add Another Event" button. (This button only appears once a list has been selected and you have saved the Add/Update Contact Event). Then select "Add/Remove Tag".
Then click the tag field and select the Tag(s) that you want to Add or Remove.
Once you have selected all of your tags, click the "Save" button at the bottom of the event.