How update 15.2.1
How update 15.2.1 on iPhone 11 Pro Max
iPhone 11 Pro Max, iOS 15
Question marked as
See the following Apple articles:
Update your iPhone, iPad, or iPod touch
https://support.apple.com/en-in/HT204204
If your iPhone, iPad or iPod touch won’t update
https://support.apple.com/en-us/HT201435
Or update via PC, see this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC
https://support.apple.com/en-gb/guide/itunes/itns3235/windows