Firebase Firestore: Update a field of a document in a subcollection while retrieving information

Question

How can I update a field to a document, while getting the the fields of that document? I am unsure how to do so given my setup. It appears that I can't use .update() with my code as I get an unresolved reference: update error.

class ReminderActivity : AppCompatActivity() {
    lateinit var reminderID: String
   
    val db = Firebase.firestore
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_reminder)

        val completeTask: Button = findViewById(R.id.complete_button)
        completeTask.setOnClickListener {
            db.collection("pets").document("vNkkDGAS1oBMpanEXhte").collection("reminders").document(iFdIMKSxJV79Mm64OHwp).get().addOnSuccessListener { document ->
                    val timeStamp = document.get("timestamp") as com.google.firebase.Timestamp
                    val millisec = timestamp.seconds * 1000 +timestamp.nanoseconds/1000000
                    val netDate = Date(millisec)
                    val frequency = document.get("frequency").toString()
                    val cal: Calendar = Calendar.getInstance()
                    cal.time = netDate
                    
                    val time = Timestamp(cal.timeInMillis)
                    document.update({timestamp: time}) //<- line not working
                }
        }
    }
}

pictures of cloud firestore database