I buy a iPad Air 2 in second hands and have iCloud account

Steps to delete iCloud account on iPhone/iPad

1. Tap on the Settings app and then scroll down to find iCloud.
2. Tap on "iCloud" to open it.
3. Scroll down until you find "Delete Account" and then tap on that.
4. Tap on "Delete" again to confirm the deletion of the iCloud account.

Restore it to the correct account

You can attach some flash drives to Apple's $29 iPad Camera Connection Kit. (If you have an iPad with a Lightning connector you'll additionally need Apple's $29 Lightning to USB Camera Adapter.) Let me underscore some. Some flash drives require more power than the iPad can provide and won't work.

For the second question, here's also a guide to help you back up iPad music to iCloud.

1. Connect your iPad to a Wi-Fi network.
2. In iOS 8 or later, tap Settings > iCloud > Backup. ...
3. Make sure that iCloud Backup is turned on.
4. Tap Back Up Now. ...
5. Make sure the backup finished by tapping Settings > iCloud > Storage > Manage Storage, and then select your iPad.

If you have a new computer, you can also backup iPad music onto it.
http://www.leawo.org/tutorial/how-to-sync-iphone-to-new-computer.html

You can get some very simple steps in this blog that will help you to make your iCloud account secure: visit here:
Tips for Offering Security to Apple iCloud Account Unveiled

plug it into a laptop/pc with itunes on . itunes will detect it and you can do a reset from there . if you got the ipad 2nd hand you might need an icloud account and pass word from previous owner or get them to take the ipad off there icloud account.

Hey dude, you need to check your icloud account and be sure of your password is correct so that you can update and can download at the appstor.
you need to go the settings and below you can see the account sync, you to sign in or to create an account on icloud

New iCloud removal service available contact accessonesolutions.com for more details. Reported lost/stolen units not accepted.

If you've tied your iPad to the iCloud account and have forgotten about that account password go to iCloud.com and click on "I forgot my Apple-ID or Password". Follow their instructions to re-activate your account. If you can't answer the questions (you defined) or have no access to the email-account assigned anymore it might get difficult.

go to settings - iCloud - and scroll to the bottom / delete account. This will remove it from the iPad.