I have x , y coordinates and I want to convert into longitude , latitude
What do x and y mean in your data?
It is difficult to help you with so little information with which to work. Your question is like “I have a tool. How is the tool used?”
Read What is a Datum and a Projection?. The datum of the x and y coordinates and the latitude and longitudes is necessary to even begin giving you an answer.
An indication of the input data accuracy and the needed output accuracy is also useful. The output accuracy can not be more accurate than the input data. You should not ask for centimeter accuracy when you are trying to locate a sports park. The computations often become much more complex as the accuracies increases.
Telling your purpose might also help.