Start by finding the "more options" icon in the upper right of your browser.
Select "Settings" from the menu.
Find the "Show Advanced Settings" link.
Find the "Clear Browsing Data" option.
Select "All Cookies and Site Data".
Use the search box to look for "usemystats".
Use the "x" icon to remove cookies for UseMyStats.
Now refresh the UseMyStats login page and you'll be all set!