Decrease of income in rectification return

I want to decrease my income by filing rectification return

We have a chance to made or not?

Here filing of orginal return is made after due date and orginal return is proceed by cpc

And another doubt is

My client is filed income tax return and it processed by CPC and it raised demand of Rs 3000 my client is not ready to pay

Can I file rectification return by decrease the income or not?

Demand raised because of House property loss is first set of LTCG and then STCG as per software and as per CPC order house property loss is first set off with STCG and then LTCG

Orginal return filed after due date

Please suggest me