Dist Acct Invalid System Hold

We have an invoice that has been on hold for a long time, and the distribution account string is no longer valid.  I released the hold and wanted to cancel the invoice (we no longer need it), but a system hold was generated "Dist Acct Invalid".  The A/P guide says that the reason for the hold must be resolved before the system hold can be released but gives no instructions on how to do so.  How do I update the distribution account string to a valid string?  The account field on the distribution screen won't allow me to update the