First, obtain a reference to the file that you want to rename. Second, invoke the renameTo() method on that reference using a File argument.

Suppose you have a file named Results.dat and you want to rename it Results_May.dat.

The code lines below show how it's done:

//Obtain the reference of the existing file
File oldFile = new File("Results.dat"); 

//Now invoke the renameTo() method on the reference, oldFile in this case
oldFile.renameTo(new File("Results_May.dat"));