Can anybody out there PLEASE tell me where i can find a simple command in LrC 10.4 to turn
a B&W negative into a positive [reverse ,invert it] or the opposite ?
Try an Invert using Curves. Then make a preset for future use. Note that if you are inverting a color neg, it may be somewhat ugly due to the Orange Mask, but for B&W, this works just fine.
Also be aware when you use Andrew suggestion some of the sliders will work backwards.
And because some of the sliders will work backwards, you may find it easier to do the inversion in Photoshop before you start editing.
Either use Edit in Photoshop from Lightroom, or do the inversion in Photoshop before importing to Lightroom.
Great ! Thanks