Hi there, since your site is hosted on WordPress.com - and not on your own server - it isn't possible to make that type of modification. Making that sort of structural change would require adding a new function to a child theme, something that's only possible if you have access to editing the theme files directly.
If you need more help with Untitled on WordPress.com, feel free to post over in the WordPress.com forums:
Tips on getting help in the WordPress.com forums: http://en.support.wordpress.com/getting-help-in-the-forums/
Differences between self-hosted WordPress and WordPress.com: