public void Modify( 
   DomainDescription domainDescription
)

{
  // Must be called within QueuedTask.Run
  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();
  }
}

{
  // Must be called within QueuedTask.Run
  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();
  }
}

{
  // Must be called within QueuedTask.Run
  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();
  }
}

{
  // Must be called within QueuedTask.Run
  void ModifyRelationshipClass(SchemaBuilder schemaBuilder, AttributedRelationshipClassDefinition attributedRelationshipClassDefinition)
  {
    AttributedRelationshipClassDescription attributedRelationshipClassDescription = new AttributedRelationshipClassDescription(attributedRelationshipClassDefinition);

    // Update the relationship split policy
    attributedRelationshipClassDescription.RelationshipSplitPolicy = RelationshipSplitPolicy.UseDefault;

    // Add field in the intermediate table
    attributedRelationshipClassDescription.FieldDescriptions.Add(FieldDescription.CreateIntegerField("RelationshipStatus"));

    // Add relationship rules based on subtypes,if available
    // Assuming origin class has subtype with code 1
    attributedRelationshipClassDescription.RelationshipRuleDescriptions.Add(
      new RelationshipRuleDescription(1, null));

    // Enqueue modify operation
    schemaBuilder.Modify(attributedRelationshipClassDescription);

    // Execute modify DDL operation
    schemaBuilder.Build();
  }
}

{
  // Must be called within QueuedTask.Run
  void ModifyAnnotationLabelAndSymbols(SchemaBuilder schemaBuilder, AnnotationFeatureClassDefinition annotationFeatureClassDefinition)
  {
    AnnotationFeatureClassDescription annotationFeatureClassDescription = new AnnotationFeatureClassDescription(annotationFeatureClassDefinition);
    IReadOnlyList<CIMLabelClass> labelClasses = annotationFeatureClassDescription.LabelClasses;

    // Adding a new annotation label class 
    List<CIMLabelClass> modifiedLabelClasses = new List<CIMLabelClass>(labelClasses);
    modifiedLabelClasses.Add(new CIMLabelClass()
    {
      Name = "RedSymbol",
      TextSymbol = new CIMSymbolReference
      {
        Symbol = new CIMTextSymbol()
        {
          Angle = 45,
          FontType = FontType.Type1,
          FontFamilyName = "Arial",
          FontEffects = FontEffects.Normal,
          HaloSize = 2.0,

          Symbol = new CIMPolygonSymbol
          {
            SymbolLayers = new CIMSymbolLayer[]
              { new CIMSolidFill { Color = CIMColor.CreateRGBColor(255, 0, 0) } },
            UseRealWorldSymbolSizes = true
          }
        },
        MaxScale = 0,
        MinScale = 0,
        SymbolName = "TextSymbol-RED"
      }
    });

    // Adding a  new symbol
    annotationFeatureClassDescription.Symbols.Add(new CIMSymbolIdentifier()
    {
      ID = 1001,
      Name = "ID_10001",
      Symbol = new CIMTextSymbol()
      {
        Angle = 43,
        FontEffects = FontEffects.Subscript,
        FontType = FontType.TTOpenType,
        FontStyleName = "Regular",
        FontFamilyName = "Tahoma",
        TextCase = TextCase.Allcaps
      }
    });

    // Modify annotation feature class 
    AnnotationFeatureClassDescription modifiedAnnotationFeatureClassDescription = new AnnotationFeatureClassDescription(annotationFeatureClassDescription.Name,
        annotationFeatureClassDescription.FieldDescriptions, annotationFeatureClassDescription.ShapeDescription,
        annotationFeatureClassDescription.GeneralPlacementProperties, modifiedLabelClasses);

    // Enqueue modify
    schemaBuilder.Modify(modifiedAnnotationFeatureClassDescription);

    // DDL execute
    schemaBuilder.Build();
  }
}

{
  // Must be called within QueuedTask.Run
  void ModifyExistingField(SchemaBuilder schemaBuilder, TableDefinition tableDefinition, string fieldNameToBeModified = "PropertyAddress")
  {
    Field field = tableDefinition.GetFields().FirstOrDefault(f => f.Name.Contains(fieldNameToBeModified));

    // Update field's alias name and length
    FieldDescription fieldDescription = new FieldDescription(field)
    {
      AliasName = "Physical Property Address",
      Length = 50
    };

    // Update the default value
    fieldDescription.SetDefaultValue("123 Main St");

    // Enqueue modify operation
    schemaBuilder.Modify(new TableDescription(tableDefinition), field.Name, fieldDescription);

    // Execute DDL
    schemaBuilder.Build();
  }
}