How do I rotate specific joints/bones using a script?

How do I rotate specific joints/bones using a script? I created a basic humanoid character in Blender, and attached a rig to it and created several animations. I created a script and set up the Animator Controller such that my character will do specific animations when I press a key.

Now, I am trying to build on top of this by creating a script that will allow me to manipulate specific joints/bones by pressing a key. I know that bones in Unity are treated as Transform objects, but I do not know the correct syntax for selecting or rotating a specific bone. My end goal is to be able to press a button and have a specific joint (e.g. the left arm) rotate.

For reference, here is my current model.

Here is the current setup I have on the Animator.

And here is the code I have on my script.

using UnityEngine; using System.Collections;

public class GraspScript : MonoBehaviour { private Animator anim; private int grasper;

  // Use this for initialization
  void Start () {
      anim = GetComponent<Animator> ();
  }
  
  // Update is called once per frame
  void Update () {
      if (Input.GetKeyDown (KeyCode.Alpha1)) {
          grasper = 1;
      } else if (Input.GetKeyDown (KeyCode.Alpha2)) {
          grasper = 2;
      } else if (Input.GetKeyDown (KeyCode.Alpha3)) {
          grasper = 3;
      } else if (Input.GetKeyDown (KeyCode.Alpha4)) {
          grasper = 4;
      } else if (Input.GetKeyDown (KeyCode.Space)) {
          grasper = 0;
      }
      anim.SetInteger ("grasp", grasper);
      Debug.Log (grasper);
  }

} Thank you so much for your help!