Hi @EyanLimi,
Welcome to the community 
You need the current dienstleistungen data in the screen where you add the update form.
You can have a screen before that edit screen and in that screen you can add a list and link the list item to the edit form screen and then you should see a option in the dropdown to select update in the form.
Check here : Trying to update collection from form but can only create new record - #2 by dilon_perera