1. Check whether the diploma has been issued: if it is not stamped, it may be because the diploma has not been issued by the school. You can contact the academic affairs office of the school or relevant departments to confirm whether the diploma has been issued.

2. Contact the school to stamp: If the diploma has been issued but not stamped, you can contact the relevant department of the school and ask if it can be stamped again. You may need to provide some relevant supporting documents and application forms.

3. Apply for academic certification: If the diploma is not stamped, you need to apply for academic certification to prove your academic qualifications. You need to provide some relevant supporting documents and application forms, and contact relevant institutions for certification.