If you want to change the primary email on your Gravatar account, you'll need to first add that email to your Gravatar profile here:
Next, we'll send you an email asking you to verify that email address. Check your email and click the link in order to activate the new address.
Once that's done, click on the new email address you added, then click "Make Primary Email" as shown in the image below. If you wish, you can also delete the email you just replaced.