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!