there's no management interfact for the tags as they are meant to be completely user defined on an ad-hoc basis.
you can use the bulk update functionality to assign the new tag and remove the old one.
a tag that is not used in the system will be automatically removed.
There is an automatic job that removes tags. It is triggered when a user does any operation that may result in making a tag unneeded (such as removing a tag from a defect).
When triggered, the system counts down 30 minutes before executing the job. If during these 30 minutes another event as mentioned above occurs, the count down resets.