I know it’s an old topic, but I like non-important problems like this. I just like my desktop to look nice! So here’s the solution for other people like me, googling this problem, and finding this topic among the first results.
Get yourself a nice .ico file. I got this image (.png) https://archeinre.deviantart.com/art/Notepad-Icon-350206145
And converted it using http://icoconvert.com/ great service, no problems. For my setup (windows 10, standard settings), 24x24 looked the best in the taskbar. (You can also convert it using gimp, but it’s quite complex. Paint should also be able to do it, but you lose transparency then.)
Once you have the .ico, go to your notepad++.exe (for me it’s in C:\Users[myusername]\AppData\Roaming\Microsoft\Windows\Start Menu\Programs) but if you have it pinned to start, you can also do Right click > more > open file location
Rightclick on notepad++ > properties > Change Icon… > Browse
Then find your .ico file. Save and close. Right click notepad++.exe again > pin to taskbar (unpin first if pinning is not an option). For me, the icon changed immediately :)
Cheers!
