HTML files can be opened in any text editor, so there is no need to convert them. Should you need it, you can simply rename the html extension to txt.
Alternatively, you could view the HTML file using any web browser and copy paste the text to a text editor. If you want to get rid of the various HTML tags, use the "paste as plain text" function instead simple paste.
Professional HTML editors like Adobe Dreamweaver or source code editors like Atom or Sublime Text are probably the best software for this so you can easily work with the HTML tags.