Open Settings
Tap General
Tap Software Update
Turn off Automatic Updates
Turn off Download iOS Updates
Turn off Install iOS Updates
If an update is already downloading, delete the update file from iPhone Storage
Go to Settings
Tap General
Tap iPhone Storage
Find the iOS update
Tap Delete Update
Keep Low Power Mode on if you want to reduce background updating
Avoid connecting to Wi-Fi if you do not want updates to download
If the update is already installed, you cannot downgrade without Apple support and compatible signed firmware