Pro Webinar - you can upload your slides at any time from within the presenter screen. If you've already uploaded slides, you'll need to remove them in order to upload a different set of slides.
For a standard (legacy) webinar - the cut-off point to upload slides or add votes is 15 minutes before the go-live time. After this, the player will be preparing to go live and will no longer accept new votes or slides. Best practice is to upload your slide deck 24 hours before the webinar goes live.
If you miss your slide deadline for a Standard Webinar, then please use the 'Get Live Support' chat option within the presenter screen. A member of the Support team will advise you on your options. You can also email firstname.lastname@example.org if you do not see the 'Get Live Support' button. Please include your slide deck and a link to the webinar in your email.