On December 31, 2018, all support for PHP 5.6 will end. This means that if a vulnerability or bug is found after support ends, there will not be a patch released by the PHP project and your site/hosting account could be easily hacked as soon as someone finds and publishes a vulnerability.
PHP is the software that runs roughly 78% of all websites (source).
PHP 5.6 was released in August 2014 and is still the most widely used version on most hosting accounts.
PHP 7.2 was released in November 2017 and has significant improvements compared to 5.6.
As with all software, it's recommended to update to the latest version as soon as an update is released. If you have logged into the backed of your Joomla site since September, you may have noticed a warning notice if your site was running PHP 5.6.
Even though it is just a warning, time is running out to ensure that your site is kept as secure as possible. We highly recommend having the necessary adjustments made as soon as possible before it's too late.
In short, to upgrade PHP versions you will need to contact your hosting provider to see what their plan is.
We've seen plenty of bad hosting companies out there. Some are even running PHP 5.2 still. Some have no plan of updating because it will break sites for thousands of customers that haven't updated their websites for years. And some have no plans on updating because they are simply lazy and do not want to deal with the time/labor until they are forced to.
If you are hosting your site(s) with us, we've already have PHP 7.2 available and enabled on the majority of the sites we host. There are a few still on PHP 5.6 that have compatibility issues due to older templates not being updated and cannot be placed on PHP 7.2 until those issues are addressed.
By January 2018 we will require any sites we host to run PHP 7.2 as we will be dropping PHP 5.6 unless you contact us with a plan/schedule to make the needed site updates within a reasonable time-frame.