If you want to change the registered email id, as of now, there is no provision in the app to change the logged in / registered email id. Nor can it be done from our end.
The only way to have a new email id is to delete the app and reinstall it again. While prompted to login, sign up with the new email id.
Also, there is currently no way to logout of the app. You have to delete the app and reinstall it again to login again.
If you have customized the app, ensure that you have taken a backup, and stored it in either iTunes or Dropbox, before deleting the app.