public static String replace(String s) {
    for (int i=0; i<s.length(); i++) {
            if (s.substring(i) == ... ) {
                //more code
            }
    }
}

StringBuilder sb = new StringBuilder("yourString");
for(int i=0;i<sb.length();i++){
    if(sb.charAt(i) == '5'){ // want to compare '5'
      sb.deleteCharAt(i);
    } else {
      sb.setCharAt(i,'6');
    }
}
String newString = sb.toString;

public class App {

public static void main(String[] args) {

    StringBuilder stringBuilder = new StringBuilder("Hello ");
    System.out.println("String :- " + stringBuilder);

    String string = "World";
    stringBuilder.insert(6, string);

    System.out.println("After insertion :- " + stringBuilder.toString());
}
}

public class App {

public static void main(String[] args) {
    StringBuilder stringBuilder = new StringBuilder("Hello World");

    System.out.println("Value:- " + stringBuilder);

    stringBuilder.delete(4, 9);
    System.out.println("After deletion :- " + stringBuilder);
}
}