Firstly, are you sure the expiry date you have is October 2021? I think the normal term is 5 years, but there have been numerous extensions over the last 18 months. The expiration date should be shown clearly on the TTP page when signed in under the Program Memberships section.

You can't start the renewal process until you are within 1 year of the expiration date.

Once you are within scope to renew you should see a green "Renew Membership" button as shown in the screenshots on this guide https://thetravelsisters.com/how-to-renew-global-entry/

You will need to do another UK check as part of the process.