Click "Edit Process Settings", go to the Extruders tab, click on the extruder you are using from the left-hand list, then set the extrusion width to manual and enter whatever value you want. Then just click OK and then "Prepare to Print" again to see the changes.
You should only need to increase your extrusion width a tiny amount, so if it was currently at 0.4mm, only increase to around 0.42mm or something similar