Click on "Log in" at the top of the screen and log in to your account. Once you are logged in, you will see your name in the same place at the top right. Click on this again, then select “My newsletter preferences” in the drop-down menu and untick the box next to “Sign up for our newsletter”. Once you've saved this, you have successfully unsubscribed from the newsletter.
Another option is to click on the unsubscribe link at the bottom of a newsletter.