Locked out of account because can’t access email or change password

My elderly mother authorized her computer several years ago with an old email address that she can no longer access.  She knows both this old email address and the password BUT the password doesn’t meet the current requirements.  When trying to log on to change the email to her current one she gets the following message “To protect the security of your account, you must reset your password. Please check your inbox for an email we just sent you with instructions for how to reset your password and log into your account.” But of course she can’t access this email!!!

She desperately needs to update her details.  How can this be done?

Thank you