Notepad++ is a text editor. It edits the whole text file.

There is no linux command header. The actual command is head, and it only displays a part of a file, not edits the file. Notepad++ is for editing, not just for displaying to the console.

If all you want is to keep just the first 10 lines in your file, then delete everything else and save (!!!but then the original file will be overwritten with just the 10 lines, and nothing else!!!).

If all you care about editing is in the first 10 lines, then just load the whole file and only edit those first 10 lines. If it really bothers you to see the other lines, you can select the lines you want to hide, and then hit View > Hide Lines (or Alt+H)