There is no upgrade that you can purchase from WordPress.com that provides FTP access and the ability to install plugins in a free hosted WordPress.com blog. See here for the comparison of the two > http://support.wordpress.com/com-vs-org/ If you require plugins you can hire a web host and set up your own WordPress.org install.
Plugins are not available at wordpress.com. There are two favors of wordpress. WordPress.com is a more controlled environment whereas wordpress.org is the location for help on using the wordpress software on self hosted sites. See http://en.support.wordpress.com/com-vs-org/ for a fuller explanation.