Mvat-cst order rectify

A Sales tax officer has passed ex-parte assessment order under CST Act for pending declarations ( C-Forms ). The dealer has not filed any appeal against it and the time limit of filing appeal also expired. In the instant case , all the declarations are already lying with dealer. So can the sales tax officer rectify/ cancel there order and do re-assessment, if dealer apply for the same ?. If no, where or whom to apply for rectification / cancellation of assessment ?The dealer has also received one more notice for the same issue by another officer. Can the other officer do reassessment and rectify order passed by the first officer? The second officer is superior in position than the first officer.?