Polyline without a Width:
Polyline with a Width:
Select the Polyline(s) who's width you want to change. When selected, nodes will appear on the Polyline as shown below.
Using the Properties Explorer, which is usually located on the right hand side of your screen (click here if not), change the width figure as highlighted below to achieve the required thickness.
Note: Due to how scaling works, the line width shown on an exported PDF may look different to how it looks on-screen, so we recommend that you create a PDF to check the width looks correct.