Power of ito to modify his previous order passed u/s143(3)

Dear Sir,

I have received an Order u/s 143(3) passed by ITO along with Form No.7. In the Form No.7 the Ld. ITO mentioned in a point "The Assessment has been completed u/s 143(1) of the I.T.Act,1961. In case you object to the assessment, you may apply to me in Form No.6-A objecting to the assessment, within one month from the date of service of this Notice of Demand". My question is that whether the Ld. ITO has the power himself to rectify or modify his previous Order passed u/s 143(3) if I file Form No.6-A before the Ld. ITO or shall I file an Appeal before CIT(Appeals) in Form No.35 against the Aseessment above Assessment Order passed by the Ld. ITO. Please let me advise in this regards.

Thanks.

C.K.Saha