Under advanced features -> Select PHP version, you can set the PHP version for your entire web hosting account. However, sometimes your WordPress, Drupal or other CMS may support a certain PHP version other than what you have set globally.
To modify the PHP version for the subdomain, you need to create a .htaccess file in the public_html folder of your subdomain. In that .htaccess file you can place the following.
SetHandler application/x-lsphp81
You may see "lsphp81" in the code. This means that the PHP version of your subdomain is set to PHP 8.1. If you wish to use PHP 7.4 or PHP 8.2, for example, you can change the code to lsphp74 and lsphp82, respectively.