All cancellation requests must be submitted in writing to the Dreamforce Help Desk here. Applicable refunds will be processed based on the time and date the request was submitted:
Full refund: Cancellation notices must be received by 11:59 p.m. PT on Friday, September 29, 2017.
50% refund: Cancellation notices must be received between 12:00 a.m. PT on Saturday, September 30, 2017, and 11:59 p.m. PT on Friday, October 27, 2017.
No refunds will be processed for cancellation notices received after 12:00 a.m. PT on Saturday, October 28, 2017.
Full cancellation and refund terms may be found in the Events Terms of Service: https://success.salesforce.com/Ev_SiteTOS