<script>

function changeUrlArg(url, arg, val){

	var pattern = arg+'=([^&]*)';

	var replaceText = arg+'='+val;

	return url.match(pattern) ? url.replace(eval('/('+ arg+'=)([^&]*)/gi'), replaceText) : (url.match('[\?]') ? url+'&'+replaceText : url+'?'+replaceText);

} 

document.write(changeUrlArg('http://www.t.com/test.php?class_id=3&id=2', 'id', 4));

</script>