I will give you one of the approach that maybe not the best (You can go research it why is that the case). But I think this is one of the simple example for you.

If I have string "hello" and I need to replace 'l' with 'y' which result to "helyo", I will use this following:

    String word = "hello";
    String toBeReplaced = "l";
    String toReplace = "y";
    
    String newWord = word.replace(toBeReplaced, toReplace);
    newWord = newWord.replaceFirst(toReplace, toBeReplaced);
    
    System.out.println(newWord);