We have detected that your server is using PHP 7.1.33 which is obsolete and does not receive official security updates from its developers. The Joomla! Project We recommend updating your site to PHP 7.2 or later and that you update security updates until at least 11-30-2020. Please ask your provider that PHP 7.2 or a later version of the sea the specific version for your site. If your provider is already prepared for PHP 7.2, PHP 7.2 active in the root and 'administration' directory of your site. Normally, you can do it through a tool in your hosting control panel, but if you are not sure it is better to ask your provider.
and when I update the php the blank site appears