I have a factory unlocked iPhone and most of the time when ever a new update or jailbreak comes, I prefer updating my iPhone manually. This process is easier and glitch free in comparison to automatic update. Though for manual update you need to download the firmware first on your desktop. Make sure you download the compatible firmware according to your iPhone version. Refer to this tutorial: How to find iPhone firmware and baseband.
In order to manually update iPhone with the firmware you have downloaded, open iTunes and connect your iPhone. Now under devices you will see your iPhone listen.
Hit the Shift button and click on update
Now iPhone will check the update with Apple server and within few minutes your iPhone will be updated manually. Don’t disconnect your iPhone in between the process.
Above method of iPhone manual update will work for windows. For Mac user combination key to browse firmware is Option Key + Update.
I hope you will find this tutorial handy and will help you to update your iPhone.