You find the slider in the parameters tab, then right click anywhere in the sliders area and select "Edit mode", then click the little gear wheel icon on the slider you want to edit (the nipple slider), then a dialog pops up and on the top right it should have a checkmark "Enable limits", uncheck that. Alternatively you can change the limit values somewhere around the middle left of that dialog, these are set to 0-100%, and you can just set them to -100-100%, or whatever you want.
Either way after you press ok you should be able to move the nipple slider left and have the nipples invert.