Hi,
In "What's new" it says "Admin PHP upgrade" which refers to PHP engine used for control panel itself. It has nothing to do with PHP used on domains.
You can check admin php version with the following command:
/usr/local/psa/admin/bin/php -v
or
/opt/psa/admin/bin/php -v
As for question of how to upgrade PHP - I believe this topic has been raised several times on this forum. Try searching it here.