If your server is running under EasyApache4 for cPanel, it now provides "MultiPHP" support whereby you can set a particular version of PHP on a site by site basis.
This guide requires EasyApache4
If you have CloudLinux, see this article: How to change site PHP version using cPanel (CloudLinux)
There is a server-wide default version of PHP, but you can specify an alternative version of PHP on individual sites.
To do this, first you must login to WHM, and then under "List Accounts" login to the cPanel for the site you wish to change:
Once logged in to the cPanel for the account, scroll down and click on "MultiPHP Manager" under the "Software" heading:
From there, on the right hand side you should be able to see the current version of PHP, as well as use the drop down menu to select a new version of PHP:
Note: Some versions of PHP may not be available, or may not be supported. Some PHP versions may have missing PHP extensions depending on server configuration. If you have any issues please contact the support team for further assistance.