How to change part of photo to black and white

Open your Photo in Photoshop, i will use below




















Double click the background layer in the Layers panel and rename the layer so that it can be editable

Create an Adjustment layer that will allow us to make changes to the photo without harming the original.

Click on created new Fill/  create new adjustment Layer and choose Hue/Saturation

When Hue/Saturation box opens, check colorize  box and then slide the Saturation slider all the way to the left. This will make your photo black and white

Create a new layer above the Adjustment layer

Choose the History Brush or simply Click Y, make sure that black is your foreground color. Then start painting the areas you want to have color show up . If you paint too much, use the eraser tool to correct

Below is final result

After

Before