Fahrenheit and Celsius are the 2 weather measurement units in the Weather app on iPhone, iPad and iPod Touch devices. To switch and change weather units on iPhone and other iOS devices, do as follows:
1. Open the Weather application from the home screen
2. Tap the 3-small-line icon on the bottom right
3. There you can add or remove cities and locations, as well as switching between Fahrenheit and Celsius units.
4. Simply tap the "C / F" button on the bottom left of the last location, to change weather units on iPhone from Celsius to Fahrenheit and from Fahrenheit to Celsius.