To change the screen saver on an iPhone, which essentially means changing the wallpaper for the Lock Screen and Home Screen, follow these steps:

1. Via Settings:
   • Open the Settings app.
   • Scroll down and tap Wallpaper.
   • Tap Add New Wallpaper.
   • Choose from categories such as Photos , People , Photo Shuffle , Live Photo , or Apple's suggested wallpaper categories like Weather, Astronomy, Emoji, or Color.
   • Customize the wallpaper if desired, then tap Add.
   • Choose Set as Wallpaper Pair to apply the same wallpaper to both Lock Screen and Home Screen, or tap Customize Home Screen to set a different wallpaper for the Home Screen.
2. Via Lock Screen:
   • From the Lock Screen, touch and hold the screen until the wallpaper gallery appears.
   • Swipe left or right to select an existing wallpaper or tap the plus icon (+) to add a new one.
   • After selecting, customize fonts, colors, and add widgets if you want.
   • Tap Add to set the wallpaper.
   • You can also link a Focus mode to a specific wallpaper or delete wallpapers from this gallery.
3. Additional Notes:
   • On iPhones with iOS 17 and later, the standby mode screen saver is linked to the Home Screen wallpaper, so changing the Home Screen wallpaper also updates the always-on display screen saver.
   • You can customize fonts, colors, and add widgets to personalize your Lock Screen further.
   • The process is quick and can be done in less than a minute.

This method covers both the traditional wallpaper change and the newer standby screen saver feature introduced in recent iOS versions