There is only a function to clear the entire LCD display, and that is in most cases visible.
Clearing only a few characters is done by writing spaces to it. After that you can write the new temperature to that part of the display.
You could also create a fixed length format of the temperature and use spaces to fill the unused characters. Then you can use just one write to the display.
It begins by designing the layout of the text on the display. Then you know where everything will be and how many characters are needed for each value.
@MichelKeijzers, I'm aware that my answer is almost the same as yours, but your answer seems to be for clearing an entire line. I would write the word "Celsius" or "°C" to the screen just once, and only update the value of the temperature.