Set metadata properties of a document | https://docs.conholdate.com/net/set-metadata-properties/

set-metadata.cs

// Constants.InputVsdx is an absolute or relative path to your document. Ex: @"C:\Docs\source.vsdx"
using (Metadata metadata = new Metadata(Constants.InputVsdx))
{
	// Set the value of each property that satisfies the predicate:
	// property contains the date/time the document was created OR modified
	var affected = metadata.SetProperties(
		p => p.Tags.Contains(Tags.Time.Created) || p.Tags.Contains(Tags.Time.Modified),
		new PropertyValue(DateTime.Now));
	Console.WriteLine("Properties set: {0}", affected);
	metadata.Save(Constants.OutputVsdx);
}

Read the complete article: https://docs.conholdate.com/net/set-metadata-properties/