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To back up user data, you need a blank SD memory card.
Start with the Garmin device turned Off.
Put the SD card into the card reader on your Garmin device (if your model has one.)
Turn On the Garmin device.
When you get to the Home screen, choose
Save To Card.
That will copy all user data to the SD card, from which it may be copied off again onto the new/replacement device.
If these instructions do not work at any point, let me know. Thanks.