Changing the PHP version used by any of the domains hosted in your account is really easy, just follow these instructions:
1 - Access your Hosting Panel
Just follow instruccions of How to access your Hosting Panel.
2 - Click on PHP Configuration
Look for and click on PHP Configuration icon.
3 - Identify the domain
You will see a listing of all domains and subdomains hosted in the account. Identify the domain on which you whish to change the PHP version.
4 - Select PHP version
Just select the desired PHP version on the dropdown .