Simple way, is the following.
Open your photo in Photoshop.
you can either use
Layer> New Adjustment Layer >Hue/Saturation,
Turning the Saturation right down.
Or use
Layers> Channel Mixer>Monochrome.
from either of the above press ok.
click on Eraser Tool, adjust brush size, opacity etc,
Start going over the areas you want to return to colour ..
There are other ways but the above is just an easy way to start...