There's no native way to change the clock font on your iPhone. However, you can use a third-party app or jailbreak your device to get the job done. There are a few different methods available, but they all require some kind of jailbreak. Some apps are free, while others are more complicated to set up and may require Cydia, which is only available with a jailbroken phone.
iOS 16 introduced new customization options for the Lock screen, including the ability to choose a different clock font and color. To use this feature, unlock your iPhone with Face ID or Touch ID, then long-press the Lock screen to bring up Customize and tap Clock. You'll see eight font options, as well as a color option.
The default font is the old thin one from iOS 15. The other options include Bold, Light, Rounded, Stencil, Serif Heavy, and Slab. You can also select a color from a palette or drag the slider to adjust its intensity.
If you prefer a more modern font style, you can download one from the Cydia store and install it with a Cydia tweak. When you restart your device, the new font should be active. Alternatively, you can use a free clock customizer like Widgetsmith or Color Widgets. Search the App Store for clock customization apps and check the reviews to ensure they're compatible with your iPhone model and iOS version. After selecting your preferred clock customization app, follow the setup instructions to apply the changes.