How to cancel software update?

What do you mean by cancel it? You can’t stop it in the middle of updating. If the iOS software has already downloaded, you can remove it in Settings>General>iPhone Storage. Swipe until you find it, then tap on it in the list of apps. You can delete it in the next screen.

Not really sure what you mean by canceling a software update. However, you can prevent automatic ones from happening. Go to Settings>General>Software Update, click Automatic Updates, and then choose off.