How can I disable the default feed on WordPress.com and use Feedburner feed instead?
I know how I can in a self-hosted WordPress site, but I do not know how to in a free WordPress.com site.
You can not disable the default WordPress.com feed but you have an option:
If you have the css upgrade, add
My Original Answer below only applies to self hosted WordPress
Claim your feed URLs at Feedburner
Then add this to your .htaccess under #END WORDPRESS
Your question is about wordpress.com, a webapplication hosting service that is using the wordpress software but wordpress.com is not fully in the scope of my answers on this Q&A site, because I don't have access to do administration and support on wordpress.com normally and therefore I'm not able to answer it with the same level of technical detail I can offer for the wordpress blogging software that can be freely downloaded.
Please ask your question to wordpress.com support. In case that does not help you much, there is also a Q&A site dedicated to hosted web-applications (like but not exclusively wordpress.com): Web Applications Stack Exchange.
I hope at least this information is somehow helpful.