[tsigel]

<div id="id1" onclick="Function1()">Click on me</div>
<script>
    var id1 = document.getElementById("id1");
    id1.outerHTML = id1.outerHTML.replace('onclick="Function1()"', 'onclick="Function2()"');

    alert(id1.outerHTML);
    alert(document.getElementById("id1").outerHTML);

    function Function1(){
        alert('Function1 is executed!');
    }

    function Function2(){
        alert('Function2 is executed!');
    }

</script>