Per discussion on User list, need section in Help to explain how the Add Reversing Transaction option works. Propose the following text:

6.7 Add Reversing Transaction

In formal accounting, transactions can never be deleted or altered. Instead, when a mistake is made, the original transaction remains in the ledger, and a reversing transaction is added to the ledger. This transaction restores the ledger to its state before the error, allowing the user to enter the transaction again correctly.

While the user can certainly enter reversing transactions manually, GnuCash includes a menu option (Transaction->Add Reversing Transaction) that can quickly create the reversing transaction for you. This option only appears when you are in an account register. When you are in the transaction that needs to be reversed, all you need to do is select Transaction->Add Reversing Transaction, and a transaction that reverses the active transaction is immediately created.

Note: the reversing transaction is immediately created in the account using the current date.