iOS 15.8.2 installing

There are about a dozen different reasons why an update will fail. Unfortunately, there are no shortcuts here to find the solution.

Work your way through the Apple Support document linked below to find the real culprit.

If your iPhone or iPad won't update - Apple Support

If you do not want to tackle this yourself, take the phone to an authorized Apple Service shop for assistance. You can find a list of authorized shops using this link:

Find Locations

Update software on your iPhone, iPad, or iPod touch

On your Windows computer, you can update software on your iPhone, iPad, or an iPod touch. 

1. Connect your Apple device to your Windows computer.
2. You can connect your device using a USB or USB-C cable.
3. In the Apple Devices app  on your Windows computer, select the device in the sidebar.
4. 
5. If you connect your device to your Windows computer using a USB or USB-C cable and don’t see the device in the sidebar, see If your iPhone, iPad, or iPod doesn’t appear in the sidebar. 
6. Select General in the sidebar.
7. 
8. Select Update.
9. Note: You may need to first select Check for Update to see if an update is available. If an update is available, the Update button appears.
10.