In order to set the thickness - before drawing the line of the wall, specify the value in centimeters in the menu on the left. The thickness is specified as integer values in centimeters. To change the thickness of an already drawn wall, call the context menu on it with the right mouse button. In the menu that appears, you can either set the value directly, or, for for more precise change control, click on the "Actions - Adjust Thickness" icon, which allows you to increase or decrease the thickness separately on each side of the wall. If you do not have these options, you are probably working in a project created in the old version of the planner (2.2), where you cannot change the wall thickness.