Do you want to change an individual line (select the line and you should see options in the ribbon at top of screen) or permanently change all the lines of a line style (format, style, select line in the top left box and away you go)?
In SE ST2 in the view tab click on styles and there you have a pulldown menu where you can select line and then you can edit the width of each line type.
If you have V20 or below look for the menu format if I remember right.
