How do I update my iPhone to iOS 14?

If your phone is running iOS 12 now – and is not offering you an upgrade to a higher major version of iOS – that would imply that it is an iPhone 5s, 6, or 6 Plus. Those phones do not support iOS 13 or higher, and cannot be updated to iOS 13 or higher.

If your device is running iOS 12.5, it is not compatible with iOS 13 or higher.