The conversion

23°F = −5°C

°C = (°F − 32) × 5/9, so (23 − 32) × 5/9 = −5.

As an outdoor air temperature, 23 °F sits in the freezing band. The other scale is on the dial beside it in the app, so a reading like this needs no conversion at all.

Convert something else

68°F

The range either side

Neighbouring values, for when the number you actually wanted was a degree or two away.

Fahrenheit to Celsius either side of 23 °F.
FahrenheitCelsius
18 °F−7.78 °C
19 °F−7.22 °C
20 °F−6.67 °C
21 °F−6.11 °C
22 °F−5.56 °C
23 °F−5 °C
24 °F−4.44 °C
25 °F−3.89 °C
26 °F−3.33 °C
27 °F−2.78 °C
28 °F−2.22 °C

The other direction

−5 °C converts back to 23 °F, which is the same relationship read the other way. Going from Celsius to Fahrenheit, multiply by 9/5 first and then add 32. Adding first is the usual mistake.

The full converter handles any value, in either direction, including kelvin, and shows the formula as it works.