We have a whole separate guide on how to configure PHP Selector on the Plesk panel. You can read it here. This how-to article explains how to setup CloudLinux PHP Selector with mod_lsapi for a website hosted on the Plesk control panel. In this guide, we assume that you already completed PHP Selector Installation.
First, you need to make sure that you have LSPHP by vendor OS PHP handler installed and enabled for your users. Logged as administrator go to Tools & Settings-> PHP Settings -> find "LSPHP by vendor OS FastCGI application" in the list. If it is disabled, you need to enable it. If you cannot find this handler in the list you will need to install native PHP on Plesk and setup the handler. Here is how you can do it:
# yum install php
# yum install liblsapi liblsapi-devel
# yum install mod_lsapi
# /usr/bin/switch_mod_lsapi --setup
The handler should be present now.
Next, log in as a user and configure this handler for a website you need to have PHP Selector running for. This is the correct setup:
That's it, go to PHP Selector and try changing the PHP versions there.