Hi @freddienaish,
Welcome to the community 
To update the department in a form or with a action you need current department data to be available in that screen! So now you can add a screen where a list of departments added and another screen where the form is added and lastly you can link the list to the screen where the form has and in the form you can select update current department!
Check here : Trying to update collection from form but can only create new record - #2 by dilon_perera
Thank you