How to install iOS 8.1.3 update using iTunes:
If you’re not getting the OTA, then you can download the iOS 8.1.3 software update from the official download links provided below and use iTunes to update your iOS device.
Before you plug in your iOS device. take a moment to make sure you are using the latest version of iTunes. Click on iTunes in the menu bar and then click on “Check for updates.”
Once iTunes is all squared away, it’s time to turn your attention to your iOS device and follow these instructions:
Step 1: Connect your iOS device to your computer using an USB cable. Wait for iTunes to open and connect to your device.
Step 2: Click on device button to the left of the iTunes Store button in the top right corner in iTunes.
Step 3: Click on the “Check for Update” directly without using holding down on the Option key or the Shift key. If the iOS 8.1.3 update is available it will automatically download and update your device to iOS 8.1.3.
Step 4: You will get a popup message informing you the new update is available. Click on Download and Update button. If it says iOS 8.1.3 is the latest version then download the appropriate firmware file using download links provided in step 5.
Step 5: Skip this step if you iOS 8.1.3 was available in Step 3 and 4.
Download the firmware file for your device for the respective devices [Note the download file is around 1.5GB to 2.0 GB depending on your device]:
After downloading the firmware file, hold the Option key (Mac) or the Shift key (Windows) and click on the “Check for Update”. Important: Remember to click on the Check for Update button, and not the Restore iPhone button to avoid restoring your iPhone and setting up your device all over again.
Step 6: If you get a popup message informing you that iTunes will update your iPhone/iPad/iPod touch to iOS 8.1.3 and will verify with Apple. Click on the Updatebutton.
Step 7: It will then show you the release notes for iOS 8.1.3. Click on the Nextbutton. Then click on the Agree button for the terms and conditions.
Step 8: iTunes will now download the firmware file (which can take a long time depending on your internet connection as it can be as big as 2.0 GB). After downloading the file, it will process it, extract the software, prepare the device for software update, and install the update.
Step 9: Do not disconnect your device until the update has finished. It can take a few minutes. Your device will be updated to iOS 8.1.3 and reboot once or twice during the process. You will see while Hello screen after it is successfully update. Follow the on-screen instructions to start using the device.
Now, enjoy with new IOS.