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:
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