Revised return of up vat 2012

Could anyone please guide me ,what is the prescribed limit of filling of revised return under up vat.?

some one say's  you can revise the return with in 6 months of date of filling..

but generally i know,it can be revised before ending of prescribed limit to filling the return for next period it means one month..ex. for june 2012 it can only be revised by 20th august .but

actually there is an error in return for the month of april or may....

please clearify this.......

thanks.