How do you undo receipt?
Steps to undo receipt --
1. On the PO , under General Tab - > Go to Related - > Documents - > Receipts

2. Select the receipt

3. Under Posted Purchase Receipt Lines - > select Manage -> Undo Receipt

You will notice a reverse transaction entry for the same quantity.

Will the previous receipt be cancelled by undoing it?
->Yes
Then by correcting the PO will it generate a new Purchase Receipt?
->Yes