Sir Actually I have downloaded iOS 15 but I don’t know why it is not installing in my phone

Click on a link below for full instructions about how to update with wi-fi or 5G, or using a computer if you have storage or connectivity issues.

Update your iPhone, iPad, or iPod touch wirelessly. Click here --> https://support.apple.com/HT204204

Update your iPhone, iPad, or iPod touch with the Finder [Catalina or later] --> https://support.apple.com/HT212185

Update your iPhone, iPad, or iPod touch with iTunes [Mac Mojave or earlier; Windows] --> https://support.apple.com/HT212186

Or this video:

"How to update your iPhone or iPad" click here --> https://youtu.be/csJAEyvvruY

If you see an error when you update or restore your iPhone, iPad, or iPod - Apple Support

If your iPhone, iPad or iPod touch won’t update - Apple Support

If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support

You must make sure the battery is at 20% or above and make sure cellular data and WiFi are working! Turn phone off and on or hit airplane! Close and open install