ok I added it. Please note that my question is more general, so to know how to do it. I tried once in the past, with an older version of xampp, to update its php by pasting in xampp folder the new php.ini, but it only returned errors. Luckily I had a backup of my .ini file and replaced it again and fixed it.
Apache is compiled with PHP as a module in XAMPP and is not possible to upgrade independently unless you know how to recompile.
Best waiting for an upgrade release or a new version release - usually there will be a forum announcement as there just has been for the first beta 1.8.0 version release and will also be published on the XAMPP web page.