Я обновил это так.
let path1 = ["channels", channel.id!, "thread"].joined(separator: "/")
let updateReference = db.collection(path1).document(message.id!)
updateReference.getDocument { (document, err) in
if let err = err {
print(err.localizedDescription)
}
else {
document?.reference.updateData([
"isRead": true
])
}
}