As with Vector3s and many other of Roblox’s custom data types, this is a more or less finalized object that can’t be changed after it is created. Throw it out and get a new one. Hopefully, this will work, the wiki page for this could use some clarification for beginner scripters. This is far more reminiscent of a more modular, strongly-typed language like Java or C#, and I’m surprised this requires so many letters.
Handles.Faces = Faces.new({
Vector3.FromNormalId(Enum.NormalId.Front),
Vector3.FromNormalId(Enum.NormalId.Top) -- Just as an example, this is how you would add more. It's just a table.
})