Welcome to the Community, Martie. Once you have completed a reconciliation for an account, you cannot change its previous ending balance directly. You'll have to undo the entire reconciliation.
Since you're using QuickBooks Online (QBO), you can only remove transaction from reconciliation one at a time, I recommend reaching out to your
accountant. QBO Accountant have the option
to bulk undo your reconciliations. They'll also be the ones to handle the adjustment if it's needed to be offset or deleted.
If you have any other concerns, please feel free to leave a comment below.