Double-click on the Arc
Arc Contextual Menu | Properties | Rendering Properties
Updated Feature: Rendering Properties
The Rendering Properties replaces the previously called Arc Style.
Clickingallows setting the default values. These values can be edited via the Preferences | Display | Arc
When using the Rendering Properties wizard, the entire set of properties are set to the selected arcs. If you just want to change one property without modifying the other ones, choose the corresponding property in the menu.
Updated Feature: Thickness Value
As of version 8.0, it is now possible to increase the thickness of the arcs up to 6.