first select the items you want to change to greyscale or select all(ctrl+A). then click recolor artwork. then you click on grayscale option. now your image is turn to a grayscale picture.
Select the objects you want to convert to grayscale or select all with "Ctrl + A". Click on "Recolor Artwork", then click on "Grayscale". Your image will be converted to greyscale.
Select the items you want to change or Selct All. Click on Recolor Artwork. Then choose the option Grayscale. Your image will now change to black and white / grayscale.