Currently it is not possible to rename film roles / folders from within darktable.
Bug: If you rename the folder in the file manager and use the DT feature "search filmrole" (context menu in the folder view of the collect module) to assign the renamed folder the new folder name doesn't show up in the UI. The change doesn't take effect until you restart darktable.
Feature: In the folder's context menu (which contains the "search filmrole" function as well) there should be an additional option "rename folder".
Ticket related to #9732