If you have only saved the ticked entry in your bank rec, then you can untick the item reconciled in error and then press Save.
If you have posted your reconciliation, and both items were in a posted reconciliation, then please follow the steps below:
post a Negative CCR or a CDR as a contra posting, to the matter on which the item was posted in error.
Reconcile by ticking off this corrective posting on your next bank rec, and press Save.