@Yong *****
Then as
@Rob***
said, your university along with the Ministry of Education will provide you all the documentation to go to the immigration office that serves the area where you live and you apply for a 90 day Non-Immigrant Type ED visa there
You will also need so many days left on your current stamp to thailand to get the process done because applications for in country Non-Immigrant type visas go under consideration for 2 weeks before you go back and get the Non-ED visa stamped into your passport.
Good Luck (y)