Admit your mistake and apologize: First of all, admit your mistake and apologize to the affected parties. Your behavior may have a negative impact on others, so your attitude should be as humble and sincere as possible.
Understand the duplicate checking mechanism: Understanding the duplicate checking mechanism can help you better understand why your paper is judged as plagiarism. This will also help you to take appropriate measures to avoid similar problems in the future.
Prove originality: If you think your paper was original in that year, you can try to provide evidence to prove your point of view. For example, you can provide drafts, notes or other evidence of that year to prove that you had finished your thesis at that time.
Revise the paper: If possible, consider making some revisions to the paper to reduce the traces of plagiarism. Please make sure that you indicate the source of all references during the revision process, and do not infringe the intellectual property rights of others.
Accept punishment: If you think your behavior violates academic ethics and norms, you should accept the corresponding punishment. This may include revoking your degree, being expelled from an academic institution or facing legal proceedings.
In short, if the paper you wrote many years ago is found to be seriously plagiarized, you should take appropriate measures to solve the problem and ensure that you avoid similar problems in the future.