Yes, it is possible to merge tags from the administrator interface. Only administrators and system animators can merge tags. You can replace a defined tag for another. The tag you specify will automatically be attached to the contents to which the initial tag was attached. This initial tag will be definitively deleted.
See our explanations on how to merge tags.
For example: I want to merge the 'knowledge plaza' tag with the 'Elium' tag. I click on the 'Merge' option next to the 'knowledge plaza' tag and indicate the 'Elium' tag as replacement.