In your browser press CTRL+SHIFT+DELETE for Windows or
CMD+SHIFT+DELETE for MacOS
A pop up similar to the below will appear (may differ slightly based on browser)
Ensure the time range is set to "ALL TIME" and that "COOKIES" is ticked, other options may be unticked and click delete data.
More instuctions can be found here
Chrome: Click here
Firefox: Click here
Edge: Click here
Safari (macOS): Click here