Your institutional ethics committee
If you have applied to your own organisation’s ethics committee, and your application has not been approved, then your first step should be to contact the administrator/coordinator of that committee. They should provide information about the appeals process. There may also be published information on the institution’s web pages.
In addition, we would strongly advise that you talk with experienced colleagues about the best way to go forward. Is there someone who could give you support and advice in preparing your appeal? Think too about whether you might be able to make a compromise in your research design in order to satisfy the committee.