1. A method for updating a model in a model predictive controller, the method comprising: assessing the deviation of the operating performance level from the desired performance level of the process plant; diagnosing the model predictive control (MPC) for the model plant mismatch (MPM), by updating the model in a model predictive controller and of the MPC thereof; wherein diagnosing said MPC comprises determining the model prediction error in relation to MPM, quantifying the MPM, and updating the model in said model predictive controller.