How do I update iOS 12.4 on my iPhone 6s

See if anything here helps.

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      

Are you asking how to get iOS 12.4? Or are you asking how to get an iOS version that is higher than 12.4?

If your iPhone really is a 6s, and not a regular 6, then you should be able to go to Settings->General-Software Update to update.

If your iPhone is running iOS 12.4.9, then it is not a 6s, but rather an iPhone 6, and cannot be updated beyond iOS 12.4.9. Its the last iOS version it supports.