Reversing the MR21 document

Hi experts

I have Posted a price changing document by MR21 for a semi finished material today.

I want to reverse it. ie. the new price i entered is wrong. i want to change it.

Please guide me how to do it.

Also explain me if suppose a material is having a released standard cost estimate for this month.

I want to change that price in between the month. When i am trying to do this system is giving error message " Already a released cost estimate is there for this material"

Thanks & Regards

Bala