If the status shows “fabrication in progress” - this means that the boards are already in fabrication and you cannot update the PCB design at this point.
If the status show “proceed with project” - This means that the boards are in pre-processing stage and pcb fabrication is not yet started. You can update the PCB design under "Update Design" option in your project dashboard.
If you face any issues while updating the design contact firstname.lastname@example.org. Please mention your project name in the subject of the mail and share the project link.