The easy way (if it's available for your account) is to go to
https://account.churchofjesuschrist.org and click on 'I forgot my username or password'. Then use the Church Membership Information option to recover the old account information. Once you have access to the old account you can just use it or you can delete it (there is button on the Personal Information page after you log in from
https://account.churchofjesuschrist.org). Once you've deleted the old account you should be able to add your MRN to your new account.
If that option doesn't work another way is to ask your ward clerk to change the email on your membership record to a current email. You should get an email to confirm that you own the email address. Once you have confirmed the email then you can use that email to recover your old account.