1. A method for updating a model in a model predictive controller of a distributed control system in a process plant, the method comprising: assessing deviation of operating performance level from desired performance level of the process plant; diagnosing model predictive control (MPC) for model plant mismatch (MPM), for updating the model in the model predictive controller and of the MPC thereof; wherein diagnosing said MPC comprises: a. determining a model prediction error in relation to MPM, wherein determining the model prediction error in relation to MPM comprises: adding white noise to set point values of controllers in the process plant; providing set points with white noise to the controllers in the process plant and to the model of the MPC; and estimating the model prediction error by calculating a difference between output values of the process plant and of the model of the MPC; and b. quantifying the MPM using non-zero lag correlation coefficients between the model prediction error and each of the manipulated variables of said MPC, said manipulated variables of the MPC being the set point values with added white noise of the controllers in the process plant; and c. updating the model in said model predictive controller for operation of the process plant based on the quantified MPM.