/admin/settings/page
Custom head code or
Custom tail code. Example:
<script>
/* Your Javascript/jQuery stuff goes in here*/
</script>
Or:
<link rel="stylesheet" href="//
www.link-to-your.css">
<script src="//
www.link-to-your.js"></script>
Usually the custom tail code section is fine to use, putting too much in the head can cause your site to go slower, depending on what you're adding.
Alternatively, edit your theme's
html_document.html file and paste between the <head> or before the closing </body> as the head and tail sections.