Managing personal Outlook.com account settings, security, and privacy
Dear Nadia,
Unfortunately, it is not possible to change the recovery email address unless you can first verify ownership of the account. Since you no longer have access to the old school email and cannot remember the password, the best option is to continue using the Microsoft Account Recovery form and provide as much information as possible (old passwords, contacts, email subjects, folders, account creation details, etc.).
If the account has been inactive for many years, it may have been permanently deleted due to Microsoft's inactivity policies, in which case it cannot be recovered. If the recovery form is unsuccessful, you can contact Microsoft Support for guidance, but they cannot bypass the account verification process or manually change the recovery email without proof of ownership.