It will take some time because of its size (5GB).
After launching ReiBoot, connect your iPhone to the computer.
First of all, download & Install Tenorshare ReiBoot on your computer.
The most common error message reported and seen by users during or after installing iOS 15 update is “iPhone stuck on Apple logo”. Many Apple device owners have reported issues with the iOS 15 update. This new version includes plenty of interesting features, but there may also be some glitches. The iOS 15 update from Apple is now available for download.