How do you change Artboard properties?

First you need to select an Artboard that you want to modify. Activate the Select tool by clicking it (or press V) and then click on the name of the Artboard that is located at the left hand side above the top Artboard edge. Once the Artboard is selected, you can change its properties in the panel at the right hand side.

Don't forget that you have to be on Design screen (so the Design tab must be active).