Either, but you need to ensure these activities happen.
Only one slide deck can be added for each webinar. For webinars with a single presenter, slides can be added by you or the presenter. If there are multiple presenters, the individual presentations need to be combined into one deck and this ‘master deck’ needs to be uploaded. It is best to decide who will own loading the master deck.
If you need to replace slides, you must upload a new presentation as you cannot replace individual slides. You and your presenters have until 15 minutes before the webinar goes live to upload slides or add votes.