Contact the location where you took the test. They should be able to assist you in getting a copy of your diploma or transcript. If you are requesting a duplicate certificate, the testing center may ask for specific forms and identification and may charge a fee.
If unsuccessful at the testing location, contact your State Department of Education. Visit your state's DOE website for direct contact information.
Obtain and complete the required paperwork, and submit with necessary identification and fees to the state's department of education.