Click the ellipsis (3 dots • • •) button next to Tags to open the Customer Tag Selector window.
This will show all Available Tags and currently Selected Tags.
You can click a tag at the top, to add it to the Selected Tags box which will then be applied to the current customer.
Move your mouse cursor on top of a tag in the Selected Tags box then click on the X next to it to unassign the Tag from the current customer.
Click Ok to save changes or Cancel to undo changes.