If you mean “When I double-click a .txt file, it opens with Notepad++ instead of MS Notepad”, then that does automatically happen when you install Notepad++. To undo it, use your Windows Operating System’s method of changing your file association – probably easiest is to right click on the .txt file, Open With, choose MS Notepad, and check “always open this type” or however it’s phrased in your exact version of Windows. (Note, these instructions aren’t unique to Notepad++ – any time you install a new file editor, it almost always changes the default association for one or more types of files, and if you don’t want that behavior, Open With is the way to re-associate it with your previous application.)
However, if, like @Lycan-Thrope assumed, you try to run notepad.exe directly (double-click on the executable, or use the Windows Run dialog and type notepad.exe) and it instead runs Notepad++, then you have done something odd at some point: probably, either you intentionally used the notepad replacement instructions – in which case you need to undo those – or you installed the “NotepadStarterPlugin”, in which case you need to follow the “Uninstallation” instructions on that plugin’s page.
