PHP versions can only be switched for web hosting, not for Domain Parking. It can be switched for each domain on the hosting separately. On a single hosting – if you have multiple domains – you can have a different PHP version on each domain.
The PHP version can be switched in Administration → My Domains → (select a domain) → PHP Settings.
The version will be switched within a few hours and can then be switched back.
PHP 5.3 was released in 2009 and support for it was officially discontinued by the developers in 2014. We are no longer supporting this version, so we strongly urge you to upgrade domains running on 5.3 to at least 5.6 if possible. In the future, this version will not be supported at all from our side.
PHP version 5.6 was released in 2014 and support for it was officially discontinued by the developers in 2018. If you can, please switch your sites to version 7.X.