Close and open new modal by clicking a link

I have two modals created with bootstrap - one for sign in and one for sign up. In each modal I have a link which I want it to refer to the other modal. E.g the sign up modal is open, registered users can click a link inside the sign up modal in order to sign in, and vice versa. How can I close the sign up div when clicking the sign in link and open the sign in div instead?

complete code in here:

http://ift.tt/1CON27Y

This is the example starting code for the sign up modal:

<div class="modal modal-backdrop fade" id="myModalJoin" tabindex="-1"
            role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
<div class="modal-dialog">
<div class="modal-contentJoin">
  <div class="modal-header">
    <button type="button" class="close" data-dismiss="modal" aria-
     label="Close"><span aria-hidden="true">
    <span class="glyphicon glyphicon-remove-sign"></span></button>
    <h2 class="modal-title join" id="myModalLabel"></h2>
    </div>
    <div class="modal-body">