主要方法：获取子字符串：

substringWithRange:

替换字符串：

stringByReplacingCharactersInRange:range 

NSString *string = @"this is my name";

eg:

    for (int i = 0; i<[string length]; i++) {
    //截取字符串中的每一个字符
        NSString *s = [string substringWithRange:NSMakeRange(i, 1)];
        NSLog(@"string is %@",s);
        if ([s isEqualToString:@"m"]) {
            NSRange range = NSMakeRange(i, 1);
    //将字符串中的“m”转化为“w”
          string =   [string stringByReplacingCharactersInRange:range withString:@"w"];

        }
    }
    NSLog(@"%@",string);