There are a few different places your email can come up, and how you change it depends on which one. Find your situation in this article.
Email address used for Cash identity verification (United States)
To change the email address used for identity verification in Cash, contact Phantom Support.
Email address used for crypto purchases
If you entered an email address when buying crypto in Phantom through a provider such as MoonPay, Topper, or Banxa, the provider is the one in charge of updating your email. Even though the purchase was made in Phantom. Contact the provider directly using the links in Onramp providers in Phantom.