Go to the dashboard and use the "Report Conflict" button. This notifies the admin, who will address the issue and potentially assign you a new desk. For more details, refer to:
Someone is using my reserved desk, what to do (reporting conflicts)
Sometimes, you may arrive at the office to find that someone is occupying the desk you’ve reserved. Dibsido makes it easy to report this conflict and find a new desk quickly. Here’s what you should do if you encounter this situation.