How to reset iCloud password to access serial number for lost iPad?

Re: "Lost ipad and can't access icloud to retrieve model number"

Your Apple Account is the account you use to access Apple services like the App Store, Apple Music, iCloud, iMessage, FaceTime, and more. It includes the email address and password you use to sign in as well as the contact, payment, and security details you use across Apple services.

See if you can verify your identity & recover your forgotten Apple Account password online: https://iforgot.apple.com/password/verify/appleid#

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Re:" ipad left on a plane, it's in lost and found but they won' release it until we give them the serial number. "

• If you have your product's original packaging, you can check the barcode to find the serial number.

• You can also find your product's serial number on the original product receipt or invoice.

Eg: If you bought the iPad from Apple, and have access to your emails:

Search for an email from Apple around the time of the iPad purchase.

Often an emailed receipt with the iPad's details is emailed to you.

• Apple support's online article: Find the serial number, EID, or IMEI on your iPhone, iPad, or iPod touch - Apple Support

Eg : says:

"Use your Apple Account

1. Go to account.apple.com in a web browser.
2. Sign in to the Apple Account that you use on your device. 
3. Choose the Devices section. To see the serial number, EID, and IMEI/MEID, select the device.

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:Use your Apple Account with a different device

Do you have a different device with iOS 10.3 or later that's signed in to your Apple Account? 

1. On the other device, go to Settings > [Your name]. 
2. Scroll down to see any devices signed in to your Apple Account. 
3. Tap the device name."

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If you find the serial number:

you could enter the serial number online to see if there's any info. available there

View Coverage - AppleCare & Warranty

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If all else fails:

You might try account recovery: How to use account recovery when you can’t reset your Apple Account password - Apple Support

Apple will try to contact you using the information you set in the old account for contacting you. Otherwise you could just be somebody trying to hack your way into another user's account. You will have to contact Apple in person and ask them if they can do anything about that.

Hers are some methods from Apple for finding your serial number.

Find the serial number of your Apple product - Apple Support

Refer to this page for Apple Support features ➔ Contact - Official Apple Support

Select from the presented options until you find a solution for your issue, or see if there is a chat or phone call contact method offered lower on the page (you sometimes have to narrow down the options multiple times before this is shown). If you do not see your issue, keep experimenting with any series of selections until you reach one that offers a chat session or a telephone call and get the representative to redirect you.

This support article has various country telephone numbers for contacting Apple for support and service ➔ Contact Apple Support - Apple Support For some countries look under “Other” listing at the end of each region. Not every country has telephone support.