The collider can be edited manually but it is often more convenient to let Unity determine the shape automatically. You can do this by dragging a sprite asset from the Project view onto the Polygon Collider 2D component in the inspector.
You can edit the polygon’s shape directly by holding down the shift key as you move the mouse over an edge or vertex in the Scene view. You can move an existing vertex by shift-dragging when the mouse is over that vertex. If you shift-drag while the mouse is over an edge then a new vertex will be created at the mouse location. You can remove a vertex by holding down the ctrl/cmd key while clicking on it.
Note that you can hide the outline of the 2D move gizmo while editing the collider - just click the foldout arrow on the Sprite Renderer component in the Inspector to collapse it.