How can I update software ios 11.3?

Is your device compatible to update to the new version? See the Apple Support article(s) below. 

iOS 14 and iPadOS 14 Compatible Devices

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

See the articles below.

Update your iOS device.        

Get help with over-the-air iOS updates          

Update – Can’t or can’t restore 

Have you tried using iTunes on a computer or Finder if running Catalina to do the update?

 Update software on iOS devices in iTunes on Mac

If you see an error when you update or restore your iPhone, iPad, or iPod      

Updating iOS: common issues and solutions