in the search box on the explorer bar at the bottom of your screen (by default), you need to type “open with”, it will bring up a menu that has the apps > settings > default apps window open for you. You will see a text box that says “search for a file type”. In that box type “txt” without the quotes, or “.txt” without the quotes. You should see an option for notepad++ in the resulting list, as long as notepad++ is installed on your system already. click on the notepad++ icon, and it will be assigned as the default app for .txt files. you should now be able to right click in any usual spot i.e. your desktop, and you should now see an option to use notepad++ under the “new” menu item from the right click menu.