Well, I just opened the file manager (the folder icon on the taskbar to the left) pressed some Location or Library link and selected Add Library (why did they change folders to libraries? To claim extra payment?)
At some point I could state where to save documents by default to the external drive. tested and worked.
Then I could delete the default library sources, C - My Documents and Public Documents, so only the external data drive is left.
It's what I would call intuitive and straight forward operated from the file manager.
By the way, my sister who started out as a secretary 30 years ago, the other day asked what I meant with 'file manager'. So sometimes we all assume too much knowledge and experience but it was straight forward.