Do you mean how to change Window’s PATH variable from pointing to Python27 to Python35? That’s a Windows question, not a Notepad++ question, and one that your original post implied you knew how to do. And 30seconds on your favorite search engine will tell you: search for “change path WindowsN” (where N is 7, 8, or 10, as appropriate for your machine).
If you mean how to change the path in the command snippet I showed, instead of typing c:\path\to\python3.exe, you would type the actual path to your installation of python3. I cannot know where that is on your computer (though c:\python35\python.exe is my first guess)
If you mean something else, you’ll have to be more explicit. Because, once again, you haven’t said how you previously ran python27 from NPP, and other than a copy/paste of my text, haven’t shown what you’ve tried to get python35 to work.