You’ll probably learn that there is more than just one problem!
One way to change the text you see displayed is to edit the csv translation files found in app/locale/en_US for the default install. Most things are run through the translation script before output, but there might be a few that aren’t. Of course if you wanted to remove translation to speed things up, that won’t work. You’ll have to change the template files.
The email templates are loaded from app/locale/en_US/template/email for the default case. Once you edit it in the backend, the new version is saved in the database. I changed the ones on disk to use static blocks for the customer service message and the style. That way if the client loads a template, they don’t get the demo text in there and all of them can be changed at once by editing the static block.
The callouts (images in left and right column) can also be made into static blocks.
The default welcome message is changed in the System - Configuration.
Some of the demo text is set as defaults in the xml files scattered throughout the code/core directories, but those values are over-ridden in other places.