To alter the Line Join Type, click on one of the choices (Mitered, Rounded, or Bevelled) under the line join heading. The selection will appear in dark grey as a visual indicator of the currently active line join type.
The image below shows what a Square would look like with a Mitered,Rounded, and Bevelled Line Join.
A line join change will be seen instantaneously when a stroke figure is selected. If no figures have been selected, the line cap change will apply to all subsequently created stroke figures.