If you're running iOS 7 or higher on your iPhone or iPad, Find My iPhone now comes with a feature called Activation Lock that prevents someone from wiping and using your device without your authorization. While it's a great theft deterrent, it can create a problem if you sell or get rid of a device before disabling Find My iPhone First. Most commonly the problem comes up with second hand iPhones that have been bought, sold, or traded. Luckily, the previous owner can deactivate Find My iPhone remotely which should then, in turn, allow you to bypass Activation Lock.
What you need to know first
Before beginning, please note that the original owner of the device will need to know the iCloud email address and password that is currently tied to the device. Without that, there is no way to bypass Activation Lock or Find My iPhone. The original owner will need to perform one of the following options, unless you know their iCloud info and can do it yourself.
How to remove a device from an iCloud account using iCloud.com
Turn the iPhone or iPad in question Off or put it in airplane mode. If the device is on and Find My iPhone has an active location, it will not allow you to delete it from your account. If someone else has the device, advise them to power it down.
From a desktop computer, go to www.icloud.com/#find and sign in using the iCloud ID that is currently tied to the iPhone or iPad in question.
You should immediately be taken to Find My iPhone, but if you aren't, just click on Find My iPhone from the Home menu.
Click on All Devices at the top of the Find My iPhone screen.
Find the device in question in the list and click on it — it must show up as Offline in order for you to be able to remove it.
On the info screen, click on Remove from Account.
One more popup will come up asking you to confirm. Click on Remove.
That's all there is to it! The new owner should now be able to restore the device and activate it just as they could if it were a brand new device. They can then link it up to their own iCloud account.
Note: Originally published, December 2013. Updated, October 2014.