Thank you for your post.
To lower the external skins what you will need to do is go to the 'Manual Overrides' section of the Wall too and edit the field called 'External Step Down (For Concrete & Joists)'. If you enter a positive number (e.g. 100mm or 4") the wall will drop down, but if you enter a negative number (e.g. -100mm or -4") it will raise the bottom of the skin/wall (this is similar to the top part of the wall with the 'External Drop Off (Positive For Eave, Negative For Parapet)' field).
Currently, the sheathing will always be generated from the same level as the bottom of the frame so if you want to lower the sheathing you'll need to manually edit it in each wall to suit. Please note: I would recommend to only do this once all additions and changes to the wall have been made as any manual edit will be removed if the wall is redrawn (this is the same throughout PlusSpec).
Would you be able to please let me know if this works for you?