Try these steps: 1. from top menu, View ► Board Planning Mode (or simply hit '1' for the keyboard shortcut) 2. from top menu, Design ► Edit Board Shape (or simply hit 'd' twice for the keyboard shortcut) (see note below) 3. left-click and drag white edit handles, which appear as you hover over a board edge
Note: For AD21 and newer, If you don't see the Edit Board Shape under the Design menu, you many need to enable Legacy.PCB.RighidFlex Preferences ► System ► General ► Advanced... button, search for "flex", then check the box in the Value column for Legacy.PCB.RighidFlex, close and restart Altium.