You did not provide a model number however the Instruction Manuals for the various Braun
Thermometers can be found here:
Not sure if this procedure is common to all but this may work otherwise find the manual specific to your model number.
Changing the temperature scale
This thermometer is shipped with the Celsius temperature scale activated. If you wish to switch to Fahrenheit ( °F), proceed as follows:
Turn on the thermometer. ( If it is already turned on, make sure it is not in memory mode.)
Press the /mem button and keep it pressed. Then press and release the activation button to switch over to the «change temperature scale» mode.
By pressing the activation button again, the Fahrenheit scale is activated, «°F» is displayed on the LCD. Each further pressing of the activation button toggles the scale between °C and °F.
Release the /mem button to return to the ready symbol . The temperature stored in memory will automatically be converted to the selected scale when displayed on the LCD.