1. Go to Launch > Report Manager and find the report you are wanting to edit.
2. Once the report has been selected go to Options > Report to launch the Report Designer.
3. Select the area of the report that has lines around it that you would like the make changes to.
4. At the top of the screen is a tool bar. The furthest-right icon is a tiny grid. Click this icon.
Tool Bar
5. From here you can make changes to the Line Style (solid, dash, dot, etc.), Weight (thin, thick, etc.), color and there are also buttons to add or remove lines on each side of the selected area. 
Line Edit Options