Revision of vat return

If company omitted to record a purchase bills agst C form in their records for  the month of June 2015 , and C form issued for that Qtr, Can the Company revise its CST  return for the qtr after issuing C form.

If yes then ok, but if No, give some suggestions in the above case.

AND also consider that the  Purchase bill is also Excisable bill . So also give suggestion regarding impact of the bill on Excise Return.