A Question about how to programmatically modify XData Block

Here is an example code to modify or create a production XData block, i guess you can adapt to you needs :

ClassMethod CreateProduction(
	package As %String = "test",
	name As %String = "AutoCreatedProduction",
	xdata As %CharacterStream) As %Status
{
  #Dim produtionClassName As %String = package _ "." _ name
  If ('$ZName(produtionClassName, 4))
  {
  Return $System.Status.Error(5001, "Invalid Production package or name.")
  }
  #Dim productionDefinition As %Dictionary.ClassDefinition
  // Check if the production already exists
  If (##class(%Dictionary.ClassDefinition).%ExistsId(produtionClassName))
  {
	// Open the production
	set productionDefinition = ##class(%Dictionary.ClassDefinition).%OpenId(produtionClassName)
  }
  Else
  {
	// Create the production definition
    set productionDefinition = ##Class(%Dictionary.ClassDefinition).%New()
  }
  //
  Set productionDefinition.Name         = produtionClassName
  Set productionDefinition.Super        = "Ens.Production"
  Set productionDefinition.ClassVersion = 25
  //
  // Check if the XData Definition already exists
  If (##Class(%Dictionary.XDataDefinition).%ExistsId(produtionClassName_"||ProductionDefinition"))
  {
	// delete the XData Definition
	$$$ThrowOnError(##Class(%Dictionary.XDataDefinition).%DeleteId(produtionClassName_"||ProductionDefinition"))
  }
  #Dim xdataDefinition As %Dictionary.XDataDefinition = ##Class(%Dictionary.XDataDefinition).%New()
  //
  Set xdataDefinition.Name = "ProductionDefinition"
  //
  Do xdataDefinition.Data.CopyFrom(xdata)
  //
  // Insert XData Definition into Production Definition
  Do productionDefinition.XDatas.Insert(xdataDefinition)
  //
  #Dim statusCode As %Status = productionDefinition.%Save()
  //
  If ($System.Status.IsError(statusCode))
  {
  Return statusCode
  }
  // Compile the production class
  return $System.OBJ.Compile(produtionClassName,"k-d")
}
ObjectScript
ObjectScript

Not so much magic in it, if you already know how to read it, the content of XData is just a Stream, which can be used both ways. Just Write to it, and %Save it. And you will probably need to compile the class

Class Demo.Test
{

XData OpenAPI [ MimeType = application/json ]
{
}

ClassMethod Update()
{
    set xdata = ##class(%Dictionary.XDataDefinition).IDKEYOpen($ClassName(), "OpenAPI")
    do xdata.Data.Write("{}")
    do xdata.%Save()
}

}
ObjectScript
ObjectScript

In VSCode, the content of XData will not appear after that, because it happened on the server, and VSCode will not see the changes, you'll need to export it manually