There is no upgrade you can purchase to do that.
The only themes you can use on any free hosted WordPress.com blog are those found here > Appearance > Themes and showcased here > http://wordpress.com/themes/
There is no FTP access to free hosted WordPress.com blogs and we cannot upload any third party themes, create child themes or our own themes or edit templates.
There is no upgrade you can purchase that changes that. Note that none of the WordPress.com upgrade plans allow you to upload your own themes or plugins or initiate advertising on your blog . If you need a site that requires a custom theme or plugins — e.g., if you want to be able to sell things directly from your site, or if you want to install discussion forums — a self-hosted WordPress site might be right for you.
WordPress.org offers free software that you can install on a web server. You can upload and install themes and plugins, run ads, and edit the database. Learn about the differences here. http://support.wordpress.com/com-vs-org/
To move to WordPress.ORG here's a brief summary of the steps to take:
1. Hire a web host http://wordpress.org/hosting/
2. Set up a WordPress.org install and equip it with a theme and plugins.
3. Export your content out of your WordPress.com blog http://en.support.wordpress.com/export/ and import it into the install.
4. Purchase a site redirect upgrade from wordpress.com
http://en.support.wordpress.com/site-redirect/ or if you have domain mapping then update the nameservers http://en.support.wordpress.com/domains/domain-management/
5. Transfer subscribers and stats. See http://en.support.wordpress.com/moving-a-blog/moving-your-subscribers/