On your browser toolbar, click the Chrome menu in the upper right:
Click More tools > Clear browsing data...
In the box that appears, click the checkbox for "Cookies and other site and plug-in data." Leave other boxes unchecked.
Use the menu at the top to select the amount of data that you want to delete. Try a shorter time period first (e.g. "the past week") and work up to "the beginning of time" if necessary.
Click Clear browsing data.
FIREFOX:
Click the menu button in the upper right:
Choose History > Clear Recent History...
Set Time range to clear to "Everything," or try a shorter time frame if you prefer.
Click on the arrow next to Details to expand the list of history items.
Select "Cookies" and make sure other items you want to keep are not selected.
Click Clear Now to clear the cookies and close the Clear Recent History window.