how to reverse the invoice registration in a purticular Purchase order invoice.

When we are raise a  Purchase invoice order 

Invoice registration made done. 

we are amend the purchase order but  We are not able to modify the unit price value.

how to reverse the invoice registration in a purticular Purchase order invoice.

After invoice registration how to change the PO unit price.

We are not able to modify the unit price value.

We are put in wrong unit price value so invoice amount showing wrongly

any one please provide the solution for this problem