How to Unreceive a Purchase Order?

It is not possible to 'unreceive' a Purchase Order.

We recommend adding a comment on the Purchase Order Master File, then returning the item by
going to the Actions button on the Item's record, and selecting Adjustments.

There is an option for Adjustment Type called 'PO Return':

This way the information will be stored correctly under that Part's history, and you
will be able to create a new Purchase Order as you wish.