The specific steps will depend on your browser, but it's usually under something like Settings > Privacy and Security. Look for 'clear cookies' or 'clear website data', etc. When you click on it, ensure it is set to clear from All Time instead of just today, etc. Note, this will likely clear all cookies for all sites, so you'll need to login again to all of your usual sites.

I see that someone already helped, but yeah most browsers you have to just clear it for all sites. There are extensions that allow you to clear them for individual sites. On Firefox, I use this one: https://addons.mozilla.org/en-US/firefox/addon/cookie-quick-manager/