Sounds like you want to do a factory default reset:
Factory default reset: Perform this reset if you wish to repurpose the AirPort Base Station or AirPort Time Capsule and want to remove all personal profiles and settings first. This reset resets the device to its state when you first purchased it. Data stored on internal or external hard drives connected to the device will not be erased. If you choose, you may manually erase the hard drive using AirPort Utility.
You can read about how to do that here:
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