Suppose you want to change the Facebook Icon URL on One Page

You can use this code to Page Header Code Injection

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.7.0/jquery.min.js"></script>
<script>
$(document).ready(function(){
    $('header#header a.icon[href*="facebook"]').attr('href','https://facebook.com/squarespace');
});
</script>

Remember to replace it with a new Facebook URL