There are two ways to refine an individual crop. You can adjust a crop by dragging or resizing any blue crop rectangle, or you can use the Advanced Editor.
The Simple Editor allows you to move the crop by clicking and dragging, or resize the crop by clicking and dragging any edge or corner.
To move the crop, simply click and drag:
To adjust a crop's dimensions, hover over its edge or any corner until your mouse changes. From any corner, you can click and drag to expand it.
To delete a crop, click the X located on the top right of any individual crop:
You will be prompted to confirm deletion.
To create a new crop, click and drag anywhere on the canvas:
The Advanced Editor allows you to more precisely adjust the crop dimensions, adjust the crop size and adjust the crop rotation to straighten it:
Personally, I think it is best to use the simple editor and store your crops in Google Photos, which has powerful tools that will likely accomplish your desired result automatically, and more precisely. For example, any trailing whitespace around the edges of your crop [like in this example], Google Photos will suggest an edit.
But if you really want to use it, here is how.
To open the advanced editor, click inside any crop and accept the prompt asking if you want to edit the crop:
The advanced editor will open. It is very large in size, so scroll using the outer right scroll bar. If you accidentally adjust the crop while trying to scroll, click the reset crop button.
To adjust an individual crop's dimensions via the Advanced Editor, click and drag on any edge:
The Reset Crop Button will revert any changes made in the Advanced Editor. It will reset the Advanced Editor's viewport to its original dimensions.
In the advanced editor, you can also apply left and right rotations to correct and straighten your crop.
The rotation adjustment is located at the bottom of the advanced editor:
To save your adjustments made in the Advanced Editor - crop location, dimensions, or rotation angle, click the "Save Changes" button:
You will notice the crops outlined in blue on the parent images will update to reflect your adjustments.
To exit the Advanced Editor, either save your changes by clicking the Save Changes button, click anywhere outside of the modal, or click the Close button: