Depending on the version of Linkbot Labs and firmware that you have, there are two different ways to reset the zero position.
If you have an older version, you have to hold down buttons A and B for about 5 seconds. After about 5 seconds, the Linkbot’s LED will blink and the new position will be remembered.
If you have a newer version, holding down buttons A and B will instead engage the Linkbot’s “Demo Mode”, where the Linkbot will continuously move its motors back and forth and slowly change the LED color. For these Linkbots, position the Linkbot to it’s zero position, and then press and hold down the A button for 5 seconds. The Linkbot will set the current position as zero, and then turn its motors for about 5 seconds as it calibrates its internal potentiometers.