You transition your site to PHP7.4 if it is supported by your host provider. But PHP7.3 is compatible to many plugins compared to the new PHP7.4 and it is not the reason if site health is not good, very far from it. My sites were running with PHP7.4 but I set it back to PHP7.3 due to incompatibility to some plugins.WordPress site health is asking to improve the site as am running an older version of PHP 7.3.20
You transition your site to PHP7.4 if it is supported by your host provider. But PHP7.3 is compatible to many plugins compared to the new PHP7.4 and it is not the reason if site health is not good, very far from it. My sites were running with PHP7.4 but I set it back to PHP7.3 due to incompatibility to some plugins.
You set in cPanel, if you are not familiar with it, ask help from your host provider support.Thanks so how do you set the PHP.