Dead pixels are when pixels on an LCD screen remain a single colour -- either red, green, blue or black -- and don't change, regardless of what's displayed on the screen. Sometimes the pixels near a dead pixel can become stuck, too. To prevent dead pixels from spreading across your phone's screen, you can fix the pixel by gently applying pressure to the area with an eraser or a similar object.
Take note of where the dead pixels are located on the screen. Turn the phone off.
Place a slightly damp cloth over the iPhone. Turn the iPhone back on.
Rub the screen where the dead pixels are located with the end of an eraser to apply pressure to the area.
Remove the eraser and cloth once the phone has fully turned on.
If you're still having trouble removing dead pixels and want to prevent spreading, you can purchase Fixing iPhone Pixels or Stuck Pixel Fix from the App Store. These apps flash bright colours on a loop to fix the pixels.