<script src="Javascript./Myjavascript.js?session="+SessionID type="text/javascript"></script>
Assuming SessionID is the asp.net session ID, you are on the right tracks but there is a problem with your implementation. If you update the js file in the middle of someone's session then their id does not change so they don't see the new version, also session IDs get re-used so they can keep their session id longer than you might think they do.
Rather than use the session id just use a simply version number that you increment each time you release
<script src="Javascript./Myjavascript.js?ver=1" type="text/javascript"></script>
The next time you update the js file change it to ver=2 and so on.