You can change your email by signing out of the app and signing up again with the new email address. Don't worry, your account information will be restored. Here's how to do it:
1) Launch Heymarket.
2) Tap 'More' in the lower right hand corner.
3) Tap 'Sign Out'.
4) You'll see a 'Welcome Back' screen. Tap 'Sign Up' at the bottom.
5) Sign up again, but with the new email address or name.