How can I add string.upper()

[noahneature]

I can't seem to understand how to input tempUnits=tempUnits.upper() so that the user can put upper and lower case F

tempUnits = input("Enter the units of temperature, C for celsius, F for fahrenheit ")

if tempUnits =='F':
    tempUnits=tempUnits.upper()
    temp = float(input("Input the temperature of water in degrees fahrenheit(°F) "))
    temp = (temp - 32) * 5/9
else:
    temp = float(input("Enter the current temperature of the water Celsius(°C) "))

if temp <= 0:
    print("Temperature in celsius must be above zero degrees, or above 32 fahrenheit")
else: