Every 3D PCB contains a sketch named Outline used to edit the board outline of the PCB. It's located in the Browser in the Sketches folder and at the beginning of the Timeline. You can edit it starting from either of these locations, or from the toolbar.
When you make changes to the Board Outline Layer in 2D PCB, the Outline Sketch will update with these changes.
When you make changes in the Outline Sketch and sync to 2D PCB, the Board Outline Layer in the 2D PCB will update with these changes. If you added constraints or projections to the Outline Sketch, the Board Outline Layer in 2D PCB will become read-only.