how do I close my account

What account are you referring to? If you mean your Apple ID, then try this Apple Support link.

Likely by mistake, you have posted your question in the support forum for the Apple routers......the AirPort Extreme, AirPort Express, and AirPort Time Capsule pictured below:

If you could provide a few more details about the account that you want to close, we may be able to direct you to the appropriate support forum for your question.