// Instead of having an expensive update function, we'll just hook playerName and run code ONLY when it changes.
    [SyncVar(hook = "UpdateName")]
    public float playerName;

    private string localName = "SomeDefaultName";
    private TextMesh textMesh;

    void Start()
    {
        textMesh = GetComponentInChildren<TextMesh>();
    }

    // This hook is run on all players when the value of playerName changes on the server.
    void UpdateName(float val)
    {
        playerName = val;

        // Update the name text for all players.
        textMesh.text = playerName;
    }

    void OnGUI()
    {
        // Only allow the owner of this player object to change this player object's name.
        // Kind of unnecessary though due to the player authority system.
        if (isLocalPlayer)
        {
            localName = GUI.TextField (new Rect (25, Screen.height - 40, 100, 30), localName);

            if (GUI.Button (new Rect (130, Screen.height - 40, 80, 30), "Change"))
            {
                CmdChangeName(localName);
            }
        }
    }

    [Command]
    void CmdChangeName (string newName)
    {
        print ("Change Player Name");
        playerName = newName;
    }