H&M Long Trench Coat

Worn a few times. Lightweight and stylish. 2 pockets in the front. No belts. The thread that holds a button on left side is broken. It can be sewn up. I just don’t know how to do it so I use a safety pin to hold it together (like in picture). Oversized form. I usually wear size L for coat. But I fit in this coat size S