You need a factory reset to change the Roku account to which your Roku is linked. To just update the email address on an existing Roku account, you only need to got to log into your Roku account and change the email address.
Log into your account at http://my.roku.com, using the email address currently on the account. It doesn't matter if that email address is now dead, you're just using it as your login name at this point. [Note you can't use the lost password option, as it can only send email to the email address currently on the account.] After you update the email address and it's been confirmed as valid, all of these will be updated: your account i.d., the email address you use to log into your Roku account, and the email address shown via Settings > System > About on your Roku devices.
