Notepad++ can edit RTF natively. If you pasted this RTF source code into Notepad++, you would be able to edit it.
{\rtf1\ansi{\fonttbl\f0\fswiss Helvetica;}\f0\pard
This is some {\b bold} text.\par
}
If you want Notepad++ to render RTF, then you would need a plugin, similar the Preview HTML plugin or Markdown plugin. In fact, if you have a standalone RTF-to-HTML converter and the 32bit Notepad++, you could easily use that Preview HTML plugin to render it.
However, if you want a WYSIWYG editor for the RTF – so it completely hides the codes, and just allows you to apply bold styles, etc, while you’re editing the text – that’s called a word processor, and isn’t what Notepad++ was built for. Your copy of Windows probably came pre-installed with Wordpad, which can handle RTF. Or get a full-featured word processor, like Microsoft Word or LibreOffice could handle it with no problem.
But you can edit teh text, and even the control codes, of a RTF just fine in Notepad++, because it’s a text-based markup (like HTML, XML, Markdown, and others)