Assuming you use Internet Explorer...
At the top of your browser, go to
TOOLS >> INTERNET OPTIONS

This will open up a new screen. There will be an option to "Delete Cookies" there. It will then come up with a comformation screen, click OK.

WARNING: this will delete every cookie/sign in you have on EVERY website. So, if you do not remember your passwords on other sites, do not do this. It will clear those.