@Alan-Robinsin ,
In general, to edit a file in administrator mode, the Notepad++ application has to be run in administrator mode. That is accomplished the same as any other Windows application, and is not unique to Notepad++.
An example sequence that worked for me
Exit all instances of Notepad++.
Run one instance as administrator (you may have to browse to the notepad++.exe and right click, run as administrator).
it should say [Administrator] at the end of the window title
File > Open, and browse to the file you want to edit. You should be able to save, if it was really an administrator-mode issue.