Step #1. Launch Settings app → Tap on General.
Step #2. Tap on Storage & iCloud Usage.
Step #3. Now tap on Manage Storage.
Step #4. Check for the iOS software version in the list and tap on it.
Step #5. Tap on Delete Update.
A menu will swipe up with one option: Delete Update.
Step #6. Tap on Delete Update.
You will no longer receive that annoying message of software update every morning. Moreover, if you want to update software in future, all you need to do is:
Launch Settings app → Tap on General → Tap on Software Update.
Now download and install the latest software update on your iOS device.