How can I turn a b/w photo into grayscale using either Lightroom or Photoshop?
This is currently not possible on the iPad, neither in Photoshop for Mobile nor in Lightroom Mobile.
You can request this as a feature here:
Photoshop for Mobile:
But how do I do it on my Mac?
Lightroom-CLASSIC always works in RGB, but you can Develop as "Black&White".
Photoshop has the MENU: Image > Mode > Greyscale.
But I ask- Why do you want "Greyscale"? Editing a photo as a Monochrome has far greater possibilities to adjust tonal values if the three 'color' channels (RGB) remain in the file.(Greyscale has only one channel)
eg. Using a B&W Adjustment Layer in Photoshop.