From the Navigation Pane or any list view, click on a folder name to open the folder. You must have "A" (Administer) rights to the folder to rename it.
From within the folder, go to Folder Options > Rename.
Type the new name and choose OK.
NOTE: Folder names can be a maximum of 1000 characters.
NOTE: A folder name cannot contain a TAB or any of these characters: \/:*?"<>|