NSMutableString *str = [NSMutableString stringWithCapacity: 5];
[str appendString:@"1-3-2"];
[str replaceCharactersInRange:NSMakeRange(2, 1) withString:@"2"];
[str replaceCharactersInRange:NSMakeRange(4, 1) withString:@"3"];

//str is "1-2-3"