I thought about this and did some searching but I think it is not possible or it should not be done because you can remove all components of Centos 7 which you installed on your hosting server, it is better to reinstall your VPS in this case so that you will have a fresh CentOS 6 to install somethings you want on it.
There isn't a supported way from CentOS in order to do it, as our colleagues have said above, the only way is to, backup, reinstall, restore. It's a bit sad, but it's the only way, good luck with this.
Downgrading OS version is not possible in CentOS, you need to backup the files and re-install the server. Also please note that upgrading from CentOS6 to CentOS 7 is impossible So you should check all your softwares should work fine in CentOS 6 before re-installing the server.