To modify an existing carrier, click on the + next to the carrier's name. By clicking on the + next to the carrier's name, it should expand two sub-sections: Settings and Advanced.
Clicking on Settings will take you to a screen that looks identical to the initial carrier setup screen. You can modify the protocol and any of the carrier's options there.
Clicking on Advanced will take you to a screen where you can specify a failover, password, message limit, maximum PIN length, and specify whether non-printable characters are sent..