You can install additional PHP versions in your ISPmanager control panel.
Log in to your control panel as root.
Login and authorization data are in your Welcome mail or the Client Area the part is "Parameters of your VPS-server"
Go to "Settings / Features"
Select the PHP version to install. You can select several versions at once using the CTRL key
Click "Install"
Confirm installation, click "OK"
Please, wait for installation completion.