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You can't deactivate license on stolen devices, but you can lock them through iCloud. On your computer, go to www.icloud.com
Login to your Apple ID or iCloud ID. Click Find iPhone.
This will show all devices associated with your Apple ID. Click your iPad Mini and click Erase or Lock button. This will make a lock on it and prevent the person from using it. DO NOT remove the iPad from your iCloud. If you remove it from iCloud, the person will be able to activate it again. Note : You can't remotely lock or erase iPod Classic as it doesn't have that feature.
I'd also recommend to change/reset your Apple ID password ***** make your account secure. You can reset it from here (go to this website from your computer) --> https://iforgot.apple.com/
Go to the above website. Use the "Get an Email" method to reset the Apple ID. Apple will send the reset instructions to the recovery email you setup with your Apple ID. It would be your email registered with Apple, like Gmail, Yahoo etc. Open your email and you will see the email from Apple. Open it and follow the instructions to reset the password.
If that didn't work, then go to this site again --> https://iforgot.apple.com/
This time use the "Answer Security Questions" and answer the security questions there to reset your password. Resetting the password ***** make your account secure.
Now to activate your new iPad Mini, all you have to do is, just sign in or resigter the new iPad Mini with your existing Apple ID or iCloud ID. Once you signed in to your existing Apple ID, you will be able to redownload all your music from online or from computer.
If you have any questions or would like any additional information, please don't hesitate to ask