If you have form post method, like:

<form name="myForm" action="/myController/DoSomething?someId=myId" method="post">

and you want myId to change to something else, then you can use this code: 

var myPom = $("form[name=myForm]").attr('action');
myPom = myPom.replace("myId", "123");
$("form[name=myForm]").get(0).setAttribute('action', myPom);