public SubtypeFieldDescription SubtypeFieldDescription {get; set;}

public void DeleteSubtypeField(SchemaBuilder schemaBuilder, FeatureClassDefinition featureClassDefinition)
{
  FeatureClassDescription featureClassDescription = new FeatureClassDescription(featureClassDefinition);

  // Set subtype field to null to remove the subtype field designation 
  featureClassDescription.SubtypeFieldDescription = null;

  schemaBuilder.Modify(featureClassDescription);
  schemaBuilder.Build();
}

public void ModifySubtypes(SchemaBuilder schemaBuilder, TableDefinition tableDefinition)
{
  TableDescription tableDescription = new TableDescription(tableDefinition);

  // Remove the first subtype from the table
  IReadOnlyList<Subtype> subtypes = tableDefinition.GetSubtypes();
  tableDescription.SubtypeFieldDescription.Subtypes.Remove(subtypes.First().GetCode());

  // Adding a new subtype, 'Utility', in the existing table
  tableDescription.SubtypeFieldDescription.Subtypes.Add(4, "Utility");

  // Assigning 'Utility' subtype as the default subtype
  tableDescription.SubtypeFieldDescription.DefaultSubtypeCode = 4;

  schemaBuilder.Modify(tableDescription);
  schemaBuilder.Build();
}