Replace/update windows.location with AJAX

I am using onchange="window.location='[login to view URL]'+[login to view URL]" to pass selected option to url and then use $_GET to pass the value in url to php. But this solution refresh page/browser. I need to update the code in order to avoid browser refresh when option is selected. Freelancer can give me the solution to add myself or I can send the form to freelancer for update and then I test. This work can be completed instantly but I pay one hour work. I need it to be done right now