To restore a high school diploma you need to write the application addressed to the Director of the school, where he was received with a request to issue a new one instead of lost, but be sure to indicate the specific cause of the loss of the certificate. In that case, if this school is in another city, then you should send your application by registered letter with acknowledgment of receipt, however, you personally have to pick it up, or to authorize friends to do it with their consent via a notarized power of attorney. In turn, the school will ask the Department (Committee, Bureau) education the form of the certificate. These forms are, in principle, should be paid from the city Treasury, but it is possible that the local authorities decided otherwise.
You will also need a copy of the certificate obtained from the police, or other documents which confirm the fact of loss (when these are absent, only one statement).
If you need to restore the certificate about the average education and also to receive the certificate of incomplete higher education, when your documents at the end of the school was handed over to the Institute you studied there for a while, then dropped out, and the documents remained at the University, then you have to apply to the University because a certificate has a long shelf life and is in the archive of this Institute.