This procedure assumes that you have already confirmed the statement page number of the transactions you need to un-reconcile. If you still need to get this information, see: Find a missing transaction in Bank Reconciliation.
To un-reconcile a bank transaction:
Go to General Ledger, Transactions, Bank Reconciliation, Old Reconcile.
Select the required bank account number from the Bank Code field.
Enter the Statement page number (there is no need to enter anything in the Opening Balance field for finding transactions only).
Click Accept (F9).
Find the transaction(s) in the Reconciled Items Table.
Highlight the line(s) in question and click Unrecon (F7).
The item will move to the Unreconciled Items table.
The transaction(s) will then be available to reconcile to the correct statement from either Reconcile or Old Reconcile.