The reset button is located just right of the Pan button (or below for Skannerz Commander). The button requires a small object such as the tip of a pen to push it down. Pressing the reset button completely erases all data on the Skanner a which point a start up screen will be presented and then a name entry screen.
The only way to change the username on a Skanner is by pressing the reset button.
Another way to reset the game is to take the batteries out and leave them out for 20 seconds, after that amount of time the screen will fade and when the batteries are put back in the game will start completely over. To take the batteries out without the game resetting first way for the device to turn itself out which takes about 60 seconds, then replace the batteries.