To change the version used by default, select it from the menu next to the Default HTML directory row, and click Save. Similarly, to change the version used by some other directory, select it from the menu next to that directory's row.
To add a custom PHP version for a new directory, enter its relative path into the text field at the bottom of the table, select a version from the menu at the bottom, and click Save. This path must be relative to public_html, like wordpress or mediawiki. There is no need for a leading / , or a full URL.
To reset a directory to use the default version of PHP, select the checkbox next to it in the table and click the Remove Selected button.