public void Modify( 
   DomainDescription domainDescription
)

public void ModifyDomain(Geodatabase geodatabase, string codedValueDomainName = "Pipe")
{
  SchemaBuilder schemaBuilder = new SchemaBuilder(geodatabase);
  CodedValueDomain codedValueDomain = geodatabase.GetDomains().First(f => f.GetName().Equals(codedValueDomainName)) as CodedValueDomain;
  CodedValueDomainDescription codedValueDomainDescription = new CodedValueDomainDescription(codedValueDomain);

  // Update domain description
  codedValueDomainDescription.Description = "Water Pipe Domain";

  // Adding code/value pair
  codedValueDomainDescription.CodedValuePairs.Add("C", "Copper");

  schemaBuilder.Modify(codedValueDomainDescription);

  // To modify the orders of coded value domain
  // schemaBuilder.Modify(codedValueDomainDescription,SortBy.Name,SortOrder.Ascending);

  schemaBuilder.Build();
}