Just bought an old AirPort Extreme. How do I know what the password is?
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There should not be one. I reset it before handing it off today. If you’d like to also reset it, you can:

1. Plug it up until you see the flashing yellow light
2. Hold down the back small pin sized button with a paper clip for about 5 seconds until you see the light flash rapidly for about 5 seconds.

All settings will have been removed and you’re ready to add it to your existing Apple network by opening Airport Utility. It will appear in the left corner of airport utility with a small “1” as a device able to be configured.