Updates an existing webinar.

Requires api token with one of the following permissions

Read, update & delete anything

Tokens with the "Act with a team member's permissions" permission
(all:delegate_to_contact_permissions scope) can also be used. Requests
made with such a token are authorized using the permissions of the
contact assigned to the token.

An expiring access token
created with the all:delegate_to_contact_permissions scope and an
authorization granting the update permission on this webinar can also
be used. The update permission also allows registering attendees and
managing the webinar's collaborators.