Open Settings
Tap General
Tap Software Update
Tap Automatic Updates
Turn off Download iOS Updates
Turn off Install iOS Updates
Go to Settings
Tap App Store
Turn off App Updates if needed
Go to Settings
Tap Wi-Fi
Use Low Data Mode if needed
Go to Settings
Tap General
Tap Background App Refresh
Turn it off if needed
Delete any downloaded update file from iPhone storage if present
Go to Settings
Tap General
Tap iPhone Storage
Find the iOS update file
Tap Delete Update
Restart the iPhone if the update keeps appearing