How can I edit the mask that is being applied to this layer?
The thing to remember is that black conceals and white reveals.
To edit the mask you will want select your mask and manipulate the white and black pixels in the mask. There are numerous techniques that allow you to manipulate a layer mask. A few of them:
- Using the brush tool to ‘paint’ on a mask
- Using the gradient tool to fade in transparency
- Using a selection and filling with a value from black to white
- Editing the properties of the mask with Windows → Properties as the mask is selected
To be more descriptive, you would apply the above techniques to your image on the canvas with the layer mask selected. To view the changes you’ve made you can Alt/Option + Click the layer mask in your layer panel. Here is an image of this technique:
Alt/Option+Clicking the layer mask to view your mask manipulations