Can't find software update on iPhone's settings

Did you look in Settings->General->Software Update?

Link-> Update your iPhone, iPad, or iPod touch - Apple Support

That's very strange.

Have you tried restarting the iPhone? Link-> Restart your iPhone - Apple Support

Is this a corporate iPhone?

Do you have an Exchange email account setup from a school or office?

Do you have a profile installed in Settings->General->VPN & Device Management?

What version of ios is it currently running?