1. Use grayscale (image > mode > grayscale)
2. Use black and white (image >adjustments > black and white)
3. Use hue and saturation (image >adjustments >hue and saturation> slide saturation towards left)
4. Use channel mixer (image >adjustments >channel mixer)
5. Use desaturate (image >adjustments > desaturate)
6. Use gradient map (image >adjustments >gradient map)>>>>
Comparing all 6 ways I prefer using channel mixer because of the nice contrast and details in the highlights are maintained