1. Assuming that you still have access to your old email address, go to the link below
2. Click Manage my Apple ID and sign in with your current iCloud ID.
3. Tap edit next to the primary email account, tap Edit, change it back to your old email account and verify it.
4. Edit the name of the account to change it back to your old email address. You can now use your current password to turn off Find My iPhone on your device.
5. Go to Settings>iCloud, tap Delete Account and choose Delete from My iDevice when asked (your iCloud data will still be in iCloud).
6. Next, go back to https://appleid.apple.com and change your primary email address and iCloud ID name back to the way it was.
7. You can now go to Settings>iCloud and sign in with your correct iCloud ID and password.
Or if you no longer have access to your old email address, you will have to contact Apple to have them reset the password so you can disable Find My iPhone and sign into your iCloud account.
You can either go to https://expresslane.apple.com, select “More Products and Services”, then “Apple ID”, then on the next page select “Other Apple ID Topics”, then “Lost or forgotten Apple ID password” and click “Continue”; or you can contact Apple Support here: http://www.apple.com/support/icloud/contact/.
The worst case is that you probably have to make an appointment with the Genius Bar at your nearest Apple store, or contact support directly.
If your device is new or you paid for AppleCare there would normally be no charge to contact support. In case they’re charging you, plead your case and see if they would waive the charge.
It’s a bit of pain, but this makes sure that your device stays secure and away from the wrong hands.