When you first run this app, it allows you to enter your credentials. These are the same credentials that you would have previously gotten from an associated web app, TARKiiZ. Subsequently, you only have to tap TRP On Ramp's icon to launch The Routing Project web application.
To change your credentials (to re-enter them), go to TRP On Ramp's "App info" screen, tap "Storage & cache" and tap the "Clear storage" button, then run TRP On Ramp again.