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This is a tutorial for the Special:RenameUser tool which only stewards and global renamers have access to (although bureaucrats on individual wikis used to perform this task).
DO check the box "Move user and talk pages (and their subpages) to new name" if it should be moved e.g. if you rename a user. If you rename a vandal/impostor the old userpage is most likely vandalism, so delete it before (or afterwards) but don't forget to delete the redirect if you decide to move anything that should not redirect to the new username.
DO NOT usurp any accounts that have made contributions.