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();
}