On our Express theme there is an "About Us" text block.
You originally set this on when you create your first event, afterwards you can change it from the "Information Settings" page.
You can change this from the manager's panel/
Settings & Options > Settings > Other Settings.
On this page you will see a long list of all the settings on your SimpleTix site. > choose: Information
On this page you will see the "About Us" textblock and you can edit it.