Unposting a converions item

we recenlty converted our system to PeopleSoft based. In this process, the items in AR were converted as paid. At this time a group of items we combined together and posted. the payemnt for these invoices have also been applied as paid by creating group amount.

Now we have a situation where we need to reverse/ unapply/ unpost one item. What are the best ways to do this?

Thanks!