If you're talking about for the domain, you need to first make sure you have PHP 5.6 installed (go to Tools & Settings, click on Upgrades and Updates, choose Add/Remove Components, click on the plus next to Web hosting, click on the plus next to PHP interpreter versions, make sure whatever PHP version you need is installed, note that it is not possible to remove PHP from OS Vendor). If it is installed then all you need to do is tell the domain that you want to use PHP 5.6 and there's 2 ways of doing it, both of which starts by clicking on Domains and clicking on the domain you want to edit. Then you can click in either PHP Settings or Hosting Settings. Where it says PHP Support, choose the version you want to use (in this case 5.6) and choose how it should be ran (NGINX, FastCGI Application served by Apache, or CGI Application served by Apache) and save your settings.