With Plesk 12.0.18 onwards, multiple PHP 5.x versions (PHP Select) can be installed and this is a new feature built by Plesk team. Via Plesk panel, without any additional third-party repositories or building PHP from sources by yourself anymore.
The new Plesk PHP packages do not conflict with or replace other PHP packages, so you can continue using your current PHP builds or third-party PHP packages.
Currently, PHP packages from the Plesk team are available on CentOS 6, CentOS 7, and Ubuntu 14. In Plesk 12.1, Plesk will extend this to Ubuntu 12, Debian 6, Debian 7, and RHEL 6, 7 operating systems
Since PLESK 10, I’ve observed that one of the most common mistake made on Plesk is the subscription expiration. This is usually neglected when PLESK is created under service provider view. Any subscription created with default settings has a 12 month validity period and will be automatically suspended by PLESK when it is expired. If you are seeing website returns a 503 page or service unavailable and this is likely the cause.
To verify your settings, sign in as admin, root or administrator to your Plesk Panel. From the menu on left hand side, select Subscription. Now, select the subscription highlighted in “RED” with an exclamation mark and choose “Customize”.
Scroll all the way down, you will find the expiration date section. To rectify, check on unlimited or select a later date and “Update”.
Remember to hit on the “Activate button” if your subscription has been suspended.
I prefer to update, install and reinstall patches of PLESK from command line.
to update and install patches
to reinstall patches