How can I change the number of layers?

If you decide at any time to change the number of physical layers or the type of manufacturing, you can do this via the Settings menu by selecting the Design Technology option and choosing the Layers tab.

Please be aware that reducing the number of layers using Delete could lead to information on the inner electrical layers being lost.

Additional reading

Creating a PCB board outline: https://www.rs-online.com/designspark/how-can-i-create-a-custom-printed-circuit-board-outline