All the answers are very good, but I'll add one more thing since you're a student:
That method should do one thing and only one thing: convert temperature from Farenheit to Celcius. That's it. No writing to files or anything else. Do that outside the method.
Why on earth are you passing Strings in and out? Temperatures are numbers; use them.
Learn the Sun Java coding standards. Variable names start with lower case letters, class names with upper case.
So your method should look like this:
public class TemperatureConverter {
private TemperatureConverter() {}
public static double celciusToFarenheit(double celcius) {
return 1.8*celcius + 32.0;
}
public static double farenheitToCelcius(double farenheit) {
return (farenheit - 32.0)/1.8;
}
}