i'm not sure if this is available with a free hosted site, but this is what i found in the support knowledge base...

How can I update the MX records for my domain name?
To update the MX records for your domain name, please do the following:

1. Log into Bravenet
2. Click Domains
3. Click Manage DNS
4. Under MX Records, click on the pencil and paper icon next to the record you want to edit
5. Enter in your new mail server and click Update Record