I neglected to state that in order to look at the HTML code you just click on the little button in the editor that says HTML, when you hover over it with your mouse it will say 'Edit HTML source' in a little pop-up.

Once you've edited the HTML click Update.

If you make a mistake and delete a bunch of text, just hit the back arrow in the Editor.

It might make the HTML code a little less daunting if you uncheck the 'Word wrap' checkbox in the top-right of the HTML source editor window - this effectively puts every tag (e.g. heading, paragraph, etc) onto a separate line.