You can do it using the User categories editor (Right-click on a category, choose Manage user categories). Select the category (all levels) you want to rename. Enter the new name in the box on the top right, being sure to include all the levels. Rinse and repeat. Press OK when done.
Warning: there is no undo. You might want to back up your library before doing it.
Notes: I will look at:
- adding the ability to copy the name in the left-hand category box to the clipboard.
- why D&D doesn't work on sub-category drags.