This is a very simple process. First you will need an HTML editor. One like MS FrontPage, Dreamweaver, or Photoshop are preferred, but not required. Any HTML editor will suffice.
If you do not have, or want do not want to pay for an HTML editor, then you can get one for free at http://nvu.com
Download and install the software, open it and use it to open the HTML (Salespage), you can now edit this just the same way you would edit a Word document using Microsoft Word.
Once you have made the changes that you are happy with, then simply "save" your work.
You have now edited your first HTML webpage. Congrats. Easy isn't it?