It is not that easy to substitute PHP in Xampp, proceed to c:/xampp/apache/conf/extra and edit httpd-xampp.conf. You will find some LoadFile and LoadModule statements which yield to PHP dlls, these have to be replaced by your own PHP version. Beware, sometimes there are not the same DLLs in a different PHP version.
You may also try to run PHP in CGI Mode instead of Module, but this requires some knowledge about the configuration of Apache and PHP. The idea of Xampp is to keep you away from this internal stuff, therefore it is not recommended to use a different PHP version than the pre-installed one.