How can I convert my mmarkdown file in different style?

Now heading 2 of my markdown file shown like this

Heading2
---

So, how can I convert it to ## Heading2

Also, the code block now is

    int gAll = 12;
    int main (int argc, char const *agc[]){
        printf("in %s gAll=%d\n", __func__, gAll);
        f();
        printf("agn %s gAll=%d\n", __func__, gAll);
        return 0;
    }
    int f(void){
        printf("in %s gAll=%d\n", __func__, gAll);
        gAll += 2;
        printf("agn %s gAll=%d\n", __func__, gAll);
        return gAll;
    }

how can I use ``` to replace the and four ?

I have tried pandoc, it can only work on heading and will let the table into a messy format.