sorry, there is no way to edit the icon from UI in the demo website.
You can edit it by editing view in the Views folder (if you have an ability to access them by downloading code to local computer or via FTP).
In the near future we're going to update demo page to be more flexible.
Anyway, you can always install Better CMS via Nuget or by downloading source code from GitHub.
These are the instructions how to install via Nuget package manager: http://www.bettercms.com/documentation/project-setup
And there are the instructions how to setup developer environment when code is downloaded from GitHub: http://www.bettercms.com/documentation/setup-develompent-environment