I want to fit the net behind the goal but I don't know how to shape the net object? Example in the picture below (how can I give the shape I want to the black drawn place?);
I guess that's Unity.
Unity has a tool called ProBuilder that allows you to do basic modeling things. This tool has an option called Probuilderize, which basically converts meshes into editable models through ProBuilder. I've never used this option so I can't talk much about it, however I know it's there and I know it does that.
This is a simple task, so I don't see why not take advantage of what you have on hand. However, I would recommend solving these types of problems in dedicated software, such as Blender. But that is already out of the scope of this answer.