replacing a character with user input

[tozymba]

Hello. I'm just beginning to learn Java and right now I'm working on simple text-based Hangman. But I am having problem replacing unknown letter with correctly guessed letter.
(i.e "_ _ _ _" unknown word is roll. User inputs "l", "_ _ _ _" this becomes "_ _ l l")
I looked at several hangman threads here but I still couldn't understand how I could replace characters.
This is what I have so far :

public static void main(String[] args) throws FileNotFoundException {
		int tries = 7;
		Scanner scan = new Scanner(System.in);
		System.out.println("=Hangman=\n");
		Hangman.random();
		System.out.println("\n\nGuess a letter:");
		String guess = scan.nextLine();
       public static void random() throws FileNotFoundException {
		Random gen = new Random();
		int ran = gen.nextInt(113810);
		String wordArr[];
		wordArr = getWords();
		String ranWord = wordArr[ran];
		System.out.println(ranWord);
		for (int i = 0; i < ranWord.length(); i++) {
			System.out.print("_ ");
		}
	}

	public static String[] getWords() throws FileNotFoundException {
		Scanner fileScan = new Scanner(new File("words.txt"));
		String words[] = new String[113809];
		String dummy = "";
		for (int i = 0; i < 113809; i++) {
			dummy = fileScan.nextLine();
			words[i] = dummy;

		}
		return words;
	}

Can anyone hint me how I could replace " _ " < these with correct letter?