To change the category of a file in #PDM you need to have PDM 2017 (or newer) version, and you must be logged in as an Admin.
Then right click on the checked in file and select "Change Category". It is also possible to select several files and change the category for all of them at the same time.
If you want to delete a category, open the administration tool, right click on the category and press delete. To complete this action properly, make sure that there are no files in the category.