WordPress.com runs the latest development branch of WordPress, so there's no need (and no way) to update your blog individually.
We don't allow plugins on WordPress.com for security reasons. Plugins are a feature only available for self-hosted blogs provided through WordPress.org.
However, we do have some really cool widgets (including a Subscribe widget). Perhaps one of those might meet your needs?
For more information on the differences between WordPress.com and WordPress.org, please see: