If you already have a student visa, your education provider will send your new CoE to the Department of Home Affairs. However, if your student visa application is still under review by the Department of Home Affairs, you should attach your new CoE to your student visa application via ImmiAccount as soon as possible.
If your new CoE is for a course at a lower AQF level, you may need to apply for a new student visa. The Australian Qualification Framework determines course levels.