As Gary indicated, that is tricky to do when the message is in HTML format.
In order to get it to look proper, it is sometimes necessary to understand
HTML and edit the HTML. Since you don't understand HTML you probably must
assume it is impossible to do in HTML format. You might be able to copy the
entire body to an editor capable of editing HTML, such as Front Page or
Word, then edit and the replace the body with the modified document, but
even that is a bit too complicated to explain here.
I think that adding a comment or a reply in the body of the email is called
replying "inline". Perhaps you can find a tutorial or something such as that
that explains how to edit HTML to reply "inline".