how to make my character walk automatically and when pressing a button change the direction

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You will need to create a float or an int hooked up to a bool. In other words, a number that is attached to a true or false statement. your code will look something like this.

 public float speed = 4f;
 public bool forward;
 
 void update()
 {
     if(Input.GetButtonDown("Fire1"))
     {
         if (forward == true)
         {
             speed = -4f;
             forward = false;
         }
         else
         {
             speed = 4f;
             forward = true;
         }
     }
 }

I don't know how much of a beginner you are, but if you don't know how to make a character move in the first place, I highly suggest you look up "Character Movement Unity3d" on youtube. There are a lot of "Introduction into coding" tutorials out there. Best of luck!