With the unit off, insert the USB. Power on the stereo. If it doesn't auto-detect, you may need to repeatedly tap the center of the screen during boot until a "detected" message appears in green. The update can take up to 15 minutes. Do not turn off the ignition during this process, as it can cause permanent failure. Safe Maintenance Alternatives
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If your unit is still working, use a tool like "Flash Tool" to backup your existing firmware before trying a new one. With the unit off, insert the USB