You can change your email by signing out of the app and signing up again with the new email address. 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.