In the top right of every page on the main website you will find "My Account". Click here and you will be taken to the login page.
From this page you can log in to your existing account as required.
Should you wish to go there directly from this page please click the below link.
Once logged in to your account click on "Personal Details", remove or add the tick to/from the "Subscribe to Newsletter" check box and click "Save Changes"
See image below: