import java.util.Scanner;
import java.io.File;

public class RenameExtension {
	public static void main(String[] args) {
		File directory = new File("C:/test/");
		File[] fileNames = directory.listFiles();
		
		Scanner myScanner = new Scanner(System.in);

		System.out.print("What is the extension you would like to change? ");
		String toChange = myScanner.next();
		System.out.print("What is the extension you would like to change it to? ");
		String theNewExt = myScanner.next();
		
		for (int i=0; i < fileNames.length; i++) {
			String thisFile = fileNames[i].toString();
			String fileMinusExt = thisFile.substring(0, thisFile.length() - 3);

			if (thisFile.endsWith(toChange)) {
				fileNames[i].renameTo(new File(fileMinusExt + theNewExt));
				System.out.println(thisFile + " was successfully renamed to " + fileMinusExt + theNewExt);
			}
		}
	}
}
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Output:

What is the extension you would like to change? txt
What is the extension you would like to change it to? dll
C:\test\Hello World.txt was successfully renamed to C:\test\Hello World.dll
C:\test\Test File 2.txt was successfully renamed to C:\test\Test File 2.dll
C:\test\TestFile1.txt was successfully renamed to C:\test\TestFile1.dll