float currentAngle, targetAngle = 0;
Vector2 buttonSize = new Vector2(120,50);
Rect buttonRect;

void Start(){
buttonRect = new Rect((Screen.width/2) - (buttonSize.x/2),(Screen.height/2) - (buttonSize.y/2),buttonSize.x,buttonSize.y);
}

void OnGUI(){
Matrix4x4 matrixBackup = GUI.matrix;
currentAngle = iTween.FloatUpdate(currentAngle, targetAngle, 3);
GUIUtility.RotateAroundPivot(currentAngle, new Vector2(buttonRect.x + (buttonRect.width/2), buttonRect.y + (buttonRect.height/2)));
if(GUI.Button(buttonRect, "Click to Flip!")){
targetAngle +=180;
}
GUI.matrix = matrixBackup;
}