how to set the value of a char at an index in a string java code example
Example: how to change single character of a string in java
//short answer: you cannot individually change any specific character
//of a String in java. You can however do this:
String s1 = "This is a String";
String s2 = s1.substring(0, 8) + "o" + s1.substring(9);
System.out.println(s2);
//Prints "This is o String", replaced the 8th character with an o