First, ensure you have the required additional PHP versions installed.
Admin > Custom Build 2.0 >
For each php1_release, php2_release, etc, set the mode. Save and Build Software to get the additional PHP versions installed. I'd restart the server here.
Then for each domain, you can set the PHP version used by that domain:-
Reseller > List Users > locate your user, and login as that user
then
Account Manager > Domain Setup > Click the domain > PHP Version Selector, and save.
Regards