Removing Apple ID from a previous owner’s account : Find My iPhone Activation Lock

Learn how to remove a device from a previous owner's Apple ID so that you can activate and use it. 

 If your iDevice has already been erased

 If the device has already been erased, but is still linked to the previous owner’s account, you'll be prompted for the previous owner’s Apple ID and password during the device setup and activation process (shown below).

If the previous owner is with you and can access the device
Ask them to enter their Apple ID and password on the Activate iPhone screen (shown above) to remove the device from their account. You can then proceed through the rest of the device setup process.
If the previous owner is not present
Contact the previous owner and ask them to follow these steps to remove the device from their account:

1. Sign in to their iCloud account at www.icloud.com/find.
2. Click All Devices to open a list of devices linked to their account, then select the device to be removed. It should show a gray dot or the word “Offline” next to the device name.
3. Click "Remove from Account" to remove the device from their account. 

After the device is removed from the previous owner’s account, turn the device off by pressing and holding the Sleep/Wake button located on the top-right side of the device. Then restart your device and proceed with device setup as you would normally.

If the device has not been erased

If the device hasn't been erased, you'll be able to access either the Home or Lock screen as shown below.

If the previous owner is with you and can access the device
Ask them to erase all content and settings by choosing Settings > General > Reset > Erase All Content and Settings. They'll need to provide their Apple ID and password. After the device is erased, you'll can proceed with the device setup process.
If the previous owner is not present
Make sure the device is powered on and connected to a Wi-Fi or cellular network. Then contact the previous owner and ask them to follow these steps to remove the device from their account:

   

1. Sign in to their iCloud account at www.icloud.com/find.
2. Click All Devices to open a list of devices linked to their account, then select the device to be removed.
3. Click the Erase button to erase all content and settings from the device. When prompted, don't enter a phone number or message. Click Next until the device is erased.
4. When the erase is complete, click "Remove from Account" to remove the device from the account

After the device is erased and removed from the account, you can proceed with the device setup process.

credit from http://support.apple.com/en-us/HT201441