You can access your Roku account to update account information at: https://my.roku.com/index
From there, log into your account and then click 'Update' below 'Account information' to edit the email address on your account.
I would recommend trying an alternate device or internet browser if you're still having troubles.
If that still doesn't clear up the issue, please send me a PM with your current Roku account email address and I'll take a closer look to see how we can assist you further.
Roku Community Manager