It's pretty easy to find information on the Internet and/or home site:
Code:
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anyway:
Spoiler: procedure

Start with the device OFF and unplugged from power
To verify the device is powered off and not in standby mode, press and hold down the power button until prompted to turn off the device
Press and hold the lower right hand corner of the display portion of the touchscreen
Power on the device while continuing to hold the display
Remove your finger from the touchscreen when "Erase all user data?" appears
Touch Yes
If text language is not in English, this is the button on the right
The device will now reset. Follow the on-screen prompts and then place your device in an area with a clear view of the sky until it acquires satellites.
Although the master reset will erase all saved favorites, any loaded custom points of interest will remain intact on the device.