If you check Settings > General > Software Update , your phone will only offer the latest signed version (iOS 12.5.8). How to Install (For Enthusiasts)
If you are a collector or hobbyist, there are unofficial methods, though they are complex and often "tethered" (requiring a computer to reboot). How do I download an older iOS version - Apple Community Download iOS 9.0 for iPhone 6
Apple uses a "signing" system to authorize software installations. Once a newer version is released, Apple stops signing older versions like iOS 9.0. If you check Settings > General > Software