Please add this code in footer.php file of your active theme just above closing body tag (</body>).
<script type="text/javascript">
jQuery(document).ready(function($) {
$(function() {
$('a[href*=#]:not([href=#])').click(function() {
if (location.pathname.replace(/^\//,'') == this.pathname.replace(/^\//,'') && location.hostname == this.hostname) {
var target = $(this.hash);
target = target.length ? target : $('[name=' + this.hash.slice(1) +']');
if (target.length) {
$('html,body').animate({
scrollTop: target.offset().top-100
}, 1000);
return false;
}
}
});
});
});
</script>
For any questions, feel free to ask me in comments.