There is no really clean way to do this -- mainly because we used the name of the homework set as the primary key in the database. (We know better now, but the fix will have to await the redesign of the database.)
The work around is not too bad. It is possible to create a set as a duplicate of an existing set and to give the new set a new name. After this is done you can delete the old set.
The student data is LOST when you duplicate the set -- only the paths to the problems are preserved. This effectively means that you cannot easily rename a set after students have worked on it.