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If you want to change how all tags of a certain kind display, for instance changing all 'samus_aran' to 'character:samus aran', you want the 'siblings' system.

If you just want to add or remove tags from a file, select it in the thumbgrid and hit F3 to open the manage tags dialog.

If you want to do really clever large-scale deletes or other operations--although be warned this is advanced--the best tool at the moment is tags->migrate tags.