Step 1：Go to Settings > iTunes&App Store on your iPhone;Step 2: Find out where the "Automatic Downloads" is and then turn off the button ‘’Updates under it.
This method is very simple but please note that the reason you are getting nagged is because the update was already downloaded and iOS wants you to install it. Sometimes, you will find you have no choose but to upgrade the new iOS version unwillingly.If you have already been forced to install the new version, you can reset your iPhone/iPad to factory settings to degrade the iOS version and then set up your iPhone without Wi-Fi. To set up your iPhone without Wi-Fi can avoid the minor chance that the newest iOS 11 may upgrade automatically again during the process of your setting up. After setting up, you can repeat the steps mentioned above to turn off iOS 11 upgrade.