<script>
function myupdate(myform) {
document.forms[myform].submit();
}
</script>
From your html element, add:
onchange="myupdate('formupdate')"
Replace "formupdate" with the actual ID of your form you want to submit.
NOTE: you will get an error message "submit is not a recognized function" if you have a submit button with a name or id of "submit". Rename your submit button to be "mysubmit" or something like that.
UPDATE: another approach that works:
onchange="this.form.submit()"