Night mode can refer to two different features on iPhone: the display/night shift and the camera night mode. Here’s how to enable each:

Night Shift (display warmth, easier on eyes)

• Open Control Center:
  • iPhones with Face ID: swipe down from the top-right corner.
  • iPhones with a Home button: swipe up from the bottom edge.
  • Tap and hold brightness, then enable Night Shift, or use the Night Shift toggle if shown.
• Or use Settings:
  • Go to Display & Brightness.
  • Tap Night Shift.
  • Turn it on manually or schedule it (Sunset to Sunrise or a custom schedule).

Night mode (camera, long-exposure in low light)

• Open Camera app.
• In low light, a moon icon (Night mode) appears. Tap it to activate, or let the iPhone automatically enable it when light is poor.
• Use the slider or on-screen controls to adjust the exposure time (longer exposure for darker scenes) if you want to manually override automatic settings.
• For some models, you can also set Night mode to stay on automatically whenever lighting requires it, and you can choose a maximum capture time.

If you’re asking about a specific iPhone model or iOS version, tell me which one you have and I’ll tailor the steps precisely.