1//short answer: you cannot individually change any specific character
2//of a String in java. You can however do this:
3
4String s1 = "This is a String";
5String s2 = s1.substring(0, 8) + "o" + s1.substring(9);
6System.out.println(s2);
7//Prints "This is o String", replaced the 8th character with an o