Follow the below steps to change your project's PHP version.
The next bundle you create will be built using the new version of PHP. It's worth noting that existing bundles will still use the version of PHP they were originally built with. Changing the value of the dropdown doesn't immediately change the PHP version running in any of your environments.
Navigate to your project's Build & Deploy page.
Scroll to the "PHP Version" section.
Select the version of PHP you want to use going forward from the dropdown.
Click "Save".