Convert the bitmap to editable shapes (Arrange > Convert to Editable Shapes. Now you can modify the shape’s outline just as you would edit a rectangle converted to editable shapes. You can add points, delete points, and curve line segments.
If you want to edit the shape of a converted bitmap to a shape such as this quick shape star, enable Snap To Objects (press the red magnet icon on the Infobar), add points to the bitmap’s rectangular outline, and drag the points and snap them to the star (or other object). Even though the bitmap is masked to the new shape, if you modify the star (or other object) you will discover the bitmap is still there in its entirety.