Where do I add my live chat code??

I have installed a live chat plugin and told to insert the below code between the head tags on each page.

Where/how do I do this??

window.$zopim||(function(d,s){var z=$zopim=function(c){z._.push(c)},$=z.s=
d.createElement(s),e=d.getElementsByTagName(s)[0];z.set=function(o){z.set.
_.push(o)};z._=[];z.set._=[];$.async=!0;$.setAttribute('charset','utf-8');
$.src='//v2.zopim.com/?2mQZKh9KQHqXUnbuXVaWBMXHJloBwibg';z.t=+new Date;$.
type='text/javascript';e.parentNode.insertBefore($,e)})(document,'script');