public static void FixDocxPaths(string inputDocxPath, string outputDocxPath)
{
   string tempDir = System.IO.Path.Combine(System.IO.Path.GetTempPath(), Guid.NewGuid().ToString());
   Directory.CreateDirectory(tempDir);

   using (var archive = ZipFile.OpenRead(inputDocxPath))
   {
	   foreach (var entry in archive.Entries)
	   {
		   string fixedEntryPath = entry.FullName.Replace("\\", "/");
		   string destinationPath = System.IO.Path.Combine(tempDir, fixedEntryPath);

		   Directory.CreateDirectory(System.IO.Path.GetDirectoryName(destinationPath));
		   entry.ExtractToFile(destinationPath, overwrite: true);
	   }
   }
   
   FixRelationships(tempDir);

   if (File.Exists(outputDocxPath)) File.Delete(outputDocxPath);

   using (var zipToCreate = ZipFile.Open(outputDocxPath, ZipArchiveMode.Create))
   {
	   foreach (var filePath in Directory.GetFiles(tempDir, "*", SearchOption.AllDirectories))
	   {
		   string relativePath = System.IO.Path.GetRelativePath(tempDir, filePath).Replace("\\", "/");
		   zipToCreate.CreateEntryFromFile(filePath, relativePath);
	   }
   }
   
   Directory.Delete(tempDir, true);
}

private static void FixRelationships(string root)
{
   foreach (var relFile in Directory.GetFiles(root, "*.rels", SearchOption.AllDirectories))
   {
	   var doc = XDocument.Load(relFile);
	   XNamespace ns = "http://schemas.openxmlformats.org/package/2006/relationships";

	   bool modified = false;
	   foreach (var rel in doc.Descendants(ns + "Relationship"))
	   {
		   var target = rel.Attribute("Target");
		   if (target != null && target.Value.Contains("\\"))
		   {
			   target.Value = target.Value.Replace("\\", "/");
			   modified = true;
		   }
	   }

	   if (modified)
		   doc.Save(relFile);
   }
}

public MainWindow()
{
   InitializeComponent();
   string inputPath = @"../../../not-working-doc.docx";
   string outputPath = @"../../../fixed.docx";
   FixDocxPaths(inputPath, outputPath);
   
   // open the fixed document here
}