Many of us would have faced this issue on our Local Dev box, which contains only the Windows Authentication by default and there are many threads, information available for this over the MSDN and any many public forums too. But even then, I just wanted to keep this info handy on this portal as well.
The simple options are
1. Open the Internet Explorer as “Run as Different User”
2. Open the site with the following URL http://siteurl/_layouts/closeConnection.aspx?loginasanotheruser=true