1. Go to the reconciliation history, not the Reconcile screen itself. From Bookkeeping > Reconcile, click History by account and select the bank or credit card account whose reconciliation you need to undo.
2. Find the reconciliation at the top of the list. QuickBooks lists completed reconciliations newest first. The one at the top is your last one, which is the one this action is built to undo cleanly.
3. Open its dropdown and select Delete. There's a small arrow or menu icon on that reconciliation's row; clicking it reveals View report and Delete options.
4. Confirm. QuickBooks will warn you this reverses the reconciliation. Once confirmed, every transaction that was checked off as part of it goes back to unreconciled status, and the beginning balance for your next reconciliation attempt resets to before that period.
5. If you need to undo one further back, not the last one, you generally have to delete the ones after it first, newest to oldest, rather than jumping straight to an older one in the middle of the list. QuickBooks' reconciliation history is a chain; removing a link in the middle without removing what came after it breaks the running beginning balance.
6. Redo the reconciliation once the underlying problem is fixed. Whatever made you need to undo it in the first place, a duplicate transaction, a wrong amount, a missing deposit, fix that first, then go back through Reconcile and run it again from scratch.