Do you mean Color to Grayscale, or you mean color to black/white 1 bit?

To turn a color picture from color to grayscale, go to menu Image/Mode/Grayscale.

Inconvinience is that some colors turn to same gray value.

The best way is Image/Adjust/Channel Mixer.
Enable Monochrome option, and mix channels to taste. Convert to Grayscale.

Regards

Michael Cervantes
MC Design Studio