As the owner of the vehicle, you must make an application to be issued with a vehicle registration certificate (Zulassungsbescheinigung Teil I) to the registration authorities which issued the registered licence plate. You can also instruct a representative to apply on your behalf with your written authorisation.
If you need an application form, you can collect it from the registration authorities and fill it out later at your own convenience. Some registration authorities allow you to download the relevant form from their website or apply online.
You can make this application in person or in writing. The registration authorities may ask you to make a declaration under oath attesting to the whereabouts of the registration certificate.
Note: You can also make this declaration under oath before a notary public. In this case, the application can also made by someone on your behalf with your written authorisation. This person must submit the declaration under oath to the authorities.