What procedures do you need to go to the third grade of Beijing primary school and transfer back to Langzhong, Sichuan?

First of all, you need to find a good receiving school in your hometown and issue a certificate of acceptance.

According to the Measures of Beijing Municipality for the Administration of School Registration of Primary and Secondary School Students

Twenty-sixth students studying in this city who transfer to other provinces and cities need to apply to the school by themselves, their parents or other legal guardians, and provide relevant certificates of receiving schools. With the consent of the school, the transfer formalities shall be handled after being audited by the district and county education administrative departments. Schools should provide academic evaluation certificates for students transferred from other provinces and cities and apply for graduation certificates.

Twenty-ninth transfer should be made within one week before the winter and summer vacation or within one week after the start of school. Transfer procedures generally do not exceed 10 working days. Transfer students need to hand over the information and files related to their school status, and implement the principle of "going with the book".