Lego has created the Robot Commander App, which allows users to connect devices (phones or tablets) to EV3 robots via bluetooth.
An iPad 2 was used when making this video
How to setup the Robot Commander App to control the Sumo Bot depends on how the user wants to control its two driving motors.
Using the Robot Commander App to control the 4-Motor Sumo Bot (building instructions can be found here) is challenging. One hand uses a Toggle box to control the large and small LEFT-side motors. The other hand uses another Toggle box to control the large and small RIGHT-side motors.