How to update from 2012 to 2014

I have 3 media servers and CASO on backup exec 2012 version.
I never see instraction how to upgrade to 2014.
What must i do first?
Upgrade first CASO? or media servers?
Can I upgrade to 2014 only CASO and one media server? will media servers(version 2012) work with CASO 2014 version?