Update a banner

Update an engagement banner. Any pending (not yet sent) deliveries for this engagement are discarded and re-queued from the updated document. While an engagement is live, its audience, targeting and scheduling fields cannot be changed (for fixed frequency engagements neither can its content); pause it first or the request fails with 409. Setting status to live starts delivery. type, project, organisation and createdBy are set server-side and ignored if supplied.