How to update my iOS version 12.57

If you mean that you can't update your 5s to iOS 12.5.7, see this support article:--> If your iPhone, iPad or iPod touch won’t update.

If you're asking why your iPhone cannot support iOS versions past 12.5.7, take a look at this compatibility chart:

Unfortunately, the iPhone 5s is no longer getting iOS updates and the latest iOS version it can run is iOS 12.5.7. The 5s was officially discontinued in 2018.