The right dialog Object > Fill and Stroke is already visible at the right edge of your screen. Goto its tab "Stroke style" and adjust stroke width. Before it select all lines you want to adjust.
There is also possible to change the line end styles. You might want to change them, too because if the stroke is wide, the sharp corners can be disturbing. Round line ends fit in any angle if they are drawn to exactly same point. Be sure that you have snapping to node points ON when you draw.