Why doesn’t it say update when I go to software update?

Is your phone compatible with the update?

iOS 13 and iPadOS are compatible with these devices - Apple Support.    

iOS 12 is compatible with these devices.    

IOS 11 is compatible with these devices    If you can upgrade to IOS 11, you can upgrade to IOS 12

IOS 10 is compatible with these devices    

What iPhone are you using? And what is your current iOS version?

Are you connected to WiFi?