If you want to upgrade your account but you use a GUEST account first you have to sign out from the GUEST account and then create an account with an email address that you will need to verify. You can create a new account from the extension or on the website: https://sessionbox.io/discover(overlay:auth/registration)
After registration and verification of the email address, you can go to: https://sessionbox.io/account/upgrade(overlay:empty) and upgrade your plan.
Do not worry about if you saved sessions under the GUEST account you can migrate them to your new account just follow this help article: https://boxlabs.freshdesk.com/a/solutions/articles/80000949216