<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', document.getElementById("change").value);

}

</script>

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