You can go to http://www.duolingo.com/#/welcome

that should take you back to the beginning of the welcome if you haven't finished it.

Use another browser (e.g. google chrome if you've logged in using firefox) or log out from your account and go to the main page again.

Same problem here, but none of the suggestions are working for me. I completed the Welcome section but would like to do it again. Is there anything I can do apart from create a new account?