In short:

Files.move(source, source.resolveSibling("newname"));

More detail:

import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardCopyOption;

The following is copied directly from http://docs.oracle.com/javase/7/docs/api/index.html:

Suppose we want to rename a file to "newname", keeping the file in the same directory:

Path source = Paths.get("path/here");
Files.move(source, source.resolveSibling("newname"));

Alternatively, suppose we want to move a file to new directory, keeping the same file name, and replacing any existing file of that name in the directory:

Path source = Paths.get("from/path");
Path newdir = Paths.get("to/path");
Files.move(source, newdir.resolve(source.getFileName()), StandardCopyOption.REPLACE_EXISTING);

String strFilePath= "C:/Users/";

public void renameFile(String strOldFileName, String strNewFileName) {
    File oldName = new File(strFilePath + "/" + strOldFileName);
    File newName = new File(strFilePath + "/" + strNewFileName);
    if (oldName.renameTo(newName)) {
        System.out.println("renamed");
    } else {
        System.out.println("Error");
    }
}