Material SphereMaterial;

// Use this for initialization
void Start()
{
    SphereMaterial = Resources.Load<Material>("Materials/man");
    if (SphereMaterial == null) Debug.Log("mat null");
    else Debug.Log("mat not null");
    Debug.Log("new Material: " + SphereMaterial.name);
    MeshRenderer meshRenderer = GetComponent<MeshRenderer>();

    // Get the current material applied on the GameObject
    Material oldMaterial = meshRenderer.material;
    Debug.Log("Applied Material: " + oldMaterial.name);
    // Set the new material on the GameObject
    meshRenderer.material = SphereMaterial;
}

public class ButtonHandler : MonoBehaviour {

    public void ButtonInteract()
    {
        Debug.Log("changing matriel ");
        // Change the material of the game object here

    }
}

public class MaterialChangeButtonHandler : MonoBehaviour {

    public MeshRenderer rendererToChange;
    public Material materialToChangeItTo;

    public void ButtonInteract()
    {
        rendererToChange.sharedMaterial = materialToChangeItTo;
    }
}