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();
}