Rather than having text appear directly on your Onescreener page as a "Main Content" item, you can choose to add an icon that generates a pop-up window with your message/ biography/ updates in it. Here's how:
Starting at your navigation page, click on LINKS. Then click on the Add new link button.
Then, check your preview options to the right of the screen to ensure that the link has appeared, and click on it to view its content. You'll see that the pop-up window contains your text. Note that this can be updated at any time!