How do we go from Fahrenheit to Celsius?

Use a converter app or math.

For a semi-accurate result: remember that 70 F is approx. 20 C.

For each 10 F added or subtracted, add or subtract 5 C (60 F = 15 C).

It's not perfect, but good enough for an instant result.

In the iPhone Weather app -- open the Weather app, tap the list icon (3 horizontal lines) at bottom right of the screen, tap the 3 dots in a circle at the top right of the next screen, tap Celsius or Fahrenheit.