Move a Rigidbody2d After Collision

I have a big problem here. I have object A in between and I have one BoxColider2d to my left and another BoxColider2d to my right. When my object moves from left to Right, i want it to revert back left after it collides with the Right BoxColider2d.

Code used to move from left to right: (Correct me if i Need another code)

public float speed;

GetComponent().AddForce(new Vector2(speed, 0));

Just like an enemy in the game Mario that moves left-Right continuously, i need the same. Below the two enemys collide with the wall and change direction. How do i achieve this? I want them to move automatically and not using Input.

Kindly help Folks :)