For standard webinars, you can add your slides through the presenter screen as seen in this video demonstration:
For Pro webinars, you can add your slides through the presenter screen by clicking the slides tab.
See this page:
* Slide upload deadline: please remember that slides must be added 15 minutes prior to the scheduled start time of standard webinars. Slides for Pro webinars can be added at any time.
* If you need to make changes to the slide deck you can upload a new presentation. You cannot replace slides on an individual basis.
* If you are one of a panel of presenters, please send your presentation to the channel owner who will combine all the presentations and then add this master deck to the webinar.