Once in the Plesk menu, find Tools & Utilities located under Server Management.
Plesk Home menu
Under Panel, click on Updates.
Click on Add Components.
Summary of Installations and Updates
Find the check box that says Different PHP interpreter versions and underneath, check for a newer version that is not currently installed. If a newer version of PHP is available, select it and click on Continue.
Addition to product components
As the upgrade runs, a log of actions being performed is shown on the page as well as a progress bar. Such upgrades should take less than 15 minutes.
Progress of installation
A message will be displayed when the upgrade has completed successfully. Click OK to return to the update panel or close the session.
For additional information, you may want to reference: