If you have jailbroken your iPhone, you will not be able to update your device via over the air update. This is because pangu and evasion jailbreaks have disabled them.
In fact if you force the OTA update on iPhone 5, iPhone 5s, iPhone 5c, iPhone 4s, iPad or iPod Touch, it will stuck in infinite “cheaking for update” look and your screen will not respond to user command.
So how to install iOS 8 on jailbroken iPhone 5, iPhone 5s, iPhone 5c and iPhone 4s
1. Connect your iPhone to the computer via USB cable and fire up iTunes. Click on Check for Update button.
2. The iTunes will automatically look for the latest update and install it.
3. You will see a pop-up message saying a new update is available. If you do not get any pop up, then download them from one the links listed below.
4. a) If you get a pop up, select download and update. The iTunes will download the latest update and install it on your iPhone. Wait you are not done yet. You will get another pop up telling you that iOS8 is available. You should select download and update again.
b) If you don’t get the pop up, hold down the Option Key (Mac) or Shift key (Windows), and click on Check for Update button. Select the firmware file you downloaded in previous step. It will verify it with the Apple and install it.