There are a few instances in which a master reset can be performed on the Forerunner, such as:
By doing this reset, all user information, workout history, and satellite data may be erased. Workouts can be backed up to training programs like Garmin Connect. Also, refer to FAQ article, "How can I back up my personal settings and profiles on my Forerunner 620, 220, 210 or 110?" if using the Forerunner 210 or 110.
To perform the master reset:
The reset on the Forerunner is successful if the device finishes powering up and goes to the setup wizard. Try the steps again if the setup wizard does not appear.
Leave the device outside with a clear view of the sky for a minimum of 20 minutes to acquire satellite data once the reset and setup wizard are complete.
*If the Forerunner 610 is not receiving a satellite signal, perform an AutoLocate before performing a master reset.
†If the Forerunner will not power down, press and hold Light until the watch powers off. This causes the watch to reboot itself.