change a letter in a string java code example

Example 1: java string replace character at position

String str = in.nextLine();	//Original String
char cr = in.next().charAt(0); // character to replace
int index = in.nextInt();	// Index where replaced
str = str.substring(0, index) + cr + str.substring(index + 1);// modified string

Example 2: java replaceall single character

.replaceAll("(?<!\\S)[^ ](?!\\S)", " ").trim()