Thanks for asking the Apple Support Communities for help with updating your iPhone. This article has the steps, and also mentions to use iTunes if you experience trouble updating wirelessly. This article also links to another article if you receive an alert updating with iTunes: Update your iPhone, iPad, or iPod touch - Apple Support
Update your device using iTunes
If you can’t update wirelessly on your iOS device, you can update manually using iTunes on a computer that you trust. If your computer is using Personal Hotspot on the iOS device that you’re updating, connect your computer to a different Wi-Fi or Ethernet network before you update.
- Install the latest version of iTunes on your computer.
- Connect your device to your computer.
- Open iTunes and select your device.
- Click Summary, then click Check for Update.
- Click Download and Update.
- If asked, enter your passcode. If you don't know your passcode, learn what to do.
If you see error messages in iTunes or need more space to update
Get help if you see error messages while updating your device with iTunes. Or if you need more space for an update, you can delete content from your device manually.
Thanks and have a wonderful day.