default MX record alteration on webfaction DNS server

I am hosting a client domain's web site on my account, along with email - the email hosting was intended to be temporary though the web site will remain. I am using webfaction dns servers with default settings for this. I would like to move the email routing elsewhere to an exchange server. While I can procure an outside dns server for this, everything I do in webfaction seems to work so well I would rather use webfaction for this. Client's records at network solutions, for example, have a very long ttl which is not good when you want to make fast changes. Webfactions records have shorter ttl which is much faster to change. So can I change the default mx 10 records for this domain on webfaction or do I have to add something that is a lower number/ higher priority - like mx5, mx6 etc.? I did not see an option to change default mx10 records that were automatically created - just an option to add new ones, such as mx5, etc.

Thanks alot.