If you are running the FreePBX Distro, downgrading asterisk is simple. Get a list of available packages with:
yum list --showduplicates asterisk13
and downgrade to a specific one with:
yum downgrade asterisk13*-13.7.0
I can't think of any reason to stop on such an old version of Asterisk, why aren't you running 13.14.1 or 13.15.0?