what I need to extend x2 visa (I have it for 3 months) for 6 months or one year

You need to prepare the following documents to extend it.
1.Valid passport;
2.Registration Form of Temporary Residence for Visitors;
3.an APPLICATION FORM with recent half-length, white background, bareheaded, full-faced 2-inch color photo. Please fill in the application form in black ink or sign pen.
4.an introduction letter issued by the host institution