When do I get the iOS app update 15
What do iOS mean
Depending on what type of iPhone you use.
If your iPhone is eligible for an update, you can check by reading this Apple article:
iPhone models compatible with iOS 15
https://support.apple.com/en-gb/guide/iphone/iphe3fa5df43/ios
If your iPhone is eligible for an update, go to: Settings -> General -> Software Update