First what you need to do is enable the Safari Develop menu, simply done in Preferences » Advanced or via the command line, whichever you prefer.
Then, click Show Web Inspector, and it'll pop up a web inspector pane below your content.
Then, you can go through the HTML (when you mouse over a section the portion on the page will highlight so you know you're in the right place) and find the text you want to edit (you may have to go pretty deep). You can then double click on the area you want to change, replace the text with your own, and hit enter.
Great trick for sending funny screenshots to friends.
[crarko adds: I tested this, and it works as described.]

