<script>

function changeXY() {

   var marker = document.getElementById('marker');
   var cx = document.getElementById("cx").value
   var cy = document.getElementById("cy").value

   alert(cx + ' ' + cy);
   marker.setAttribute('cx', cx);
   marker.setAttribute('cy', cy);

}

</script>

CX: <input type="text" id="cx" onchange="changeXY();">
CY: <input type="text" id="cy" onchange="changeXY();">