To remove a transfer tag on a transaction, you can first click into the category tag on the transaction.
You'll click the X to unselect the tag. You can either choose a new tag, or leave it untagged. Next, hit the "Tag" button. It will ask you to confirm that you're unlinking the transfer to another account. You can confirm that you would like to unlink it, which will remove the transfer tag and assign it to the new category you selected.