If all you need to do is update the email address attached to your Roku account, a factory reset may be overkill. Save Factory reset as a last resort as you'll be setting the Roku up all over again from scratch, including linking it to your Roku account, adding your channels, and logging back into any channels whose subscriptions require logins.
Email address change procedure:
Log into https://my.roku.com/account/edit using your old email address as your login name -- it doesn't matter if it's not still an active email address, at this point it's just a login name. (If you don't know the password, though, you are out of luck because the "Forgot password" option can only send a reminder to the address currently on record, so if you can't access that email address you're probably stuck with having to do a factory reset.)
Once logged in, update the email address and press the Save Changes button. When confirmed, this will not only change the email address shown via Settings > System > About on any Roku device(s) linked to that account, it will also update your login name for that account.
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