This question has been asked many times and saw this on the DC Embassy page, hope it helps...
DC Embassy:
Flight cancelled? Please contact us for a revised COE. Travelers who have received COEs but have to change plan due to flight cancellation should write to the embassy / consulates-General. They will be given special access to their old accounts on the COE application platform to amend flight/ASQ details and re-apply for a new COE.