Hi heldrida,
Welcome to Affinity Forums
The default units depend on the type of document you chose when you create it. Currently there's no way to set new defaults for these types of documents.
If you want to change the overall document units select the View Tool (the hand icon) go to the context toolbar and select the units you want to use from the Units dropdown.
If you want to change the units for the lines/shapes stroke's width go to Affinity Designer/Photo Preferences, User Interface section and untick Show lines in points.
One thing you can do however is set up a customised document as you wish save it somewhere on your computer and use it as a template.
