I am trying to change the color of my background which is a green screen to black. How can I change the color to black? I am using a Macbook air laptop.
You don't need to delete the background. A new adjustment layer will be automatically created above your original layer once you choose to add the adjustment layer.
You can choose green or cyan and use the eyedroppers. After using the eyedroppers the dialog box may change what it says, depending on which color it considers closest to what you've eyedroppered.
In the layers panel, click the Adjustment icon and choose Hue/Saturation.
Double-click the Hue/Saturation to open the dialog box and choose Green from the list.
Use the Eyedropper tool in the Hue/Saturation dialog box to click on the green area. Use the Eyedropper with a '+' to click and drag over more of the green area.
Then drag the Lightness slider all the way to the left.
The model has a slight green reflection in the hair, glasses, skin. You could go over those areas with the Sponge tool set to Desaturate.
The green reflection can pretty much be avoided in the first place by photographing the model further away from the background green screen.