You can change your email address via the ‘Account’ button in the app’s main menu. From the home screen, go to ‘Account’ at the bottom of the menu, then click on your name and select ‘Edit Profile’. Press ‘Menu’ to open the drop-down menu, click on ‘Account Security’ and then press ‘Edit Email Address’. Follow the steps on the screen to enter and save your new email address.
Are you receiving an error message stating that the change cannot be made, for example because the new email address already exists? If so, it is possible that an account is already linked to that email address. Log in to that existing account, delete the account (via Account > Delete account), and then try changing your email address again.