T. Kaltnekar's answer is fine for this program, but Default Programs Editor can do this for any program and any context menu action for any number of extensions.
Open the program, click
File Type Settings. The search box, in addition to searching extensions and descriptions, supports limited advanced queries, in the format
<verb>:<application>, where "verb" means the item on the file type's context menu, and "application" is the program that item launches when clicked.
So in your case, you'd search for
open:notepad to get a list of all file types that Open with Notepad:
You can then select all the items you want out of that list (Ctrl+Click, Shift+Click or Click+Drag like normal) then hit
On the Context Menu page, click the item you want to edit (
Open in this case) and click
Edit Selected Command. Then you're on this page, where you put in the path to the program of your choice. For you, this is Notepad++ (
%1 is the parameter you pass the program so that it opens the file you've clicked. It's a common Windows thing):
Save Context Menu, and you're done.