Hi @suitangxiaoyi,
Which code are you talking about?
ArduSub is the firmware that runs on the autopilot, and it receives control instructions from an external controller - ArduSub should not need to be modified in order to control the vehicle.
If you have a positioning sensor you should be able to set up missions using QGroundControl’s Plan View.
Controlling the path with your own external program should also be possible, but the possibilities depend on what you have available. This post (and the posts it links to) may be of interest 