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!

All of your joints are considered as GameObject in your rig hierarchy (in the inspector).
You can apply a transform to these GameObject :

// You store the transform of your specific gameObject
//Rig should be metarig.001 in your case (avoid using “.” as name for your gameObjects)
Transform jointObj = Rig.transform.FindChild(boneName);

//we set the rotation you want on your joint (we transform the euler angles in quaternion)
Vector3 rot = new Vector3(rot.x + 90f, rot.y, rot.z);
jointObj.rotation = Quaternion.Euler(rot);

keep in mind that your rotation transform inherit from the parents
if you want to change only the local rotation, use → jointObj.localRotation = Quaternion.Euler(rot);