public void Modify( 
   TableDescription tableDescription
)

  public void RemoveFieldTableSnippet(Geodatabase geodatabase)
  {
      // Removing all fields from 'Parcels' table except following 
      // Tax_Code
      // Parcel_Address

      // The table to remove fields
      string tableName = "Parcels";

      TableDefinition tableDefinition = geodatabase.GetDefinition<TableDefinition>(tableName);
      IReadOnlyList<Field> fields = tableDefinition.GetFields();

      // Existing fields from 'Parcels' table
      Field taxCodeField = fields.First(f => f.Name.Equals("Tax_Code"));
      Field parcelAddressField = fields.First(f => f.Name.Equals("Parcel_Address"));

      FieldDescription taxFieldDescription = new FieldDescription(taxCodeField);
      FieldDescription parcelAddressFieldDescription = new FieldDescription(parcelAddressField);

      // Fields to retain in modified table
      List<FieldDescription> fieldsToBeRetained = new List<FieldDescription>()
{
  taxFieldDescription, parcelAddressFieldDescription
};

      // New description of the 'Parcels' table with the 'Tax_Code' and 'Parcel_Address' fields
      TableDescription modifiedTableDescription = new TableDescription(tableName, fieldsToBeRetained);

      SchemaBuilder schemaBuilder = new SchemaBuilder(geodatabase);

      // Remove all fields except the 'Tax_Code' and 'Parcel_Address' fields
      schemaBuilder.Modify(modifiedTableDescription);
      schemaBuilder.Build();
  }

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();
}