You can do it with jQuery pretty easily:
$(window).scroll(function(){
if ($(window).scrollTop() >= target) { // change target to number
$("#el").css('position', 'relative');
}
});
or pure JavaScript:
window.onscroll = function(){
if(window.scrollY >= target) { // change target to number
document.getElementById('el').style.position = 'relative';
}
};