just open the library, where the tag is defined, go to “All tags”, mark the tag and click on the trash bin icon. Or use the right mouse button on the tag and chose “Delete tag” from the menu.
It’s not super intuitive at first, but you can usually manage these from the main file list. If you click the ‘Tags’ icon or column in the web interface, it should show you all the tags currently in use for that library. From there, you should see a small ‘x’ or a delete option next to the tag name. Just keep in mind that once you remove it from all files, the tag itself usually disappears from the suggestions list too.