Once you associate an email with your Stack Exchange OpenID, you can't change it, this is status-bydesign. Says Kevin Montrose:
If you've lost access to the e-mail you registered with initially, create a new openid.stackexchange account and either add it to your Stack Overflow account (via your profile page, if you're still logged in) or create a new Stack Overflow account and email us via the Contact link so we can merge the two accounts.
From: How do I change the email address associated with my Stack Exchange OpenID?
Rather, you have to create a new SE OpenID here: https://openid.stackexchange.com - confirm via email - then go to your Super User profile, click My Logins on the upper right corner, between flair and apps. A popup appears - then click add more logins. From there, click the Stack Exchange OpenID option and type your credentials, click confirm and you're good to go.