The transaction you are editing has been reconciled. Saving your changes could put you out of balance the next time you try to reconcile. How to resolve this??
It is more a warning then an error to resolve. It is telling you that because you have finalised a reconciled period, by making changes to a transaction within this reconciled period the result may impact the balance, therefore throwing out your reconciliation or future reconciled periods.
Overall you will need to decide that if you want make the change details of a reconciled transaction, confirming first the changes you make match the banking statement of your reconciled period e.g. Opening & Closing balances