The normal way to rename or move a directory is to rename or move each file within it as described in Outside. Then check out with the ‘-P’ option, as described in Removing directories.
If you really want to hack the repository to rename or delete a directory in the repository, you can do it like this:
$ cd $CVSROOT/parent-dir $ mv old-dir new-dir
If someone had a working copy the CVS commands will cease to work for him, until he removes the directory that disappeared inside the repository.
It is almost always better to move the files in the directory instead of moving the directory. If you move the directory you are unlikely to be able to retrieve old releases correctly, since they probably depend on the name of the directories.