To edit a folder select the action "Edit folder" in the folder's action menu (the three dots on the right edge of the folder entry). After that a dialog will open in which you can modify the folder's name and its description and you can select another folder type. If the folder type is changed permissions for other Stud.IP users on that folder may change, too.

After you clicked "Save" on the bottom of the dialog all changes made on the folder will be saved.