You can delete cookies already stored on your computer; for example:
(a) in Internet Explorer (version 10), you must manually delete cookie files (you can find instructions for doing so at http://support.microsoft.com/kb/278835);
(b) in Firefox (version 24), you can delete cookies by clicking “Tools”, “Options” and “Privacy”, then selecting “Use custom settings for history”, clicking “Show Cookies”, and then clicking “Remove All Cookies”; and
(c) in Chrome (version 29), you can delete all cookies by accessing the “Customise and control” menu, and clicking “Settings”, “Show advanced settings” and “Clear browsing data”, and then selecting “Delete cookies and other site and plug-in data” before clicking “Clear browsing data”.