Обновление данных в QLabel без участия пользователя

Как вариант. Создаем дополнительный поток, который мониторит серийный порт. Периодически отправляем данные из дополнительного потока в основной поток для отображения.

from PyQt5 import QtCore, QtGui, QtWidgets


class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(800, 600)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.textEdit = QtWidgets.QTextEdit(self.centralwidget)
        self.textEdit.setGeometry(QtCore.QRect(430, 0, 361, 71))
        self.textEdit.setObjectName("textEdit")
        self.pushButton_2 = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton_2.setGeometry(QtCore.QRect(20, 170, 261, 41))
        self.pushButton_2.setObjectName("pushButton_2")
        self.pushButton_3 = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton_3.setGeometry(QtCore.QRect(20, 250, 261, 41))
        self.pushButton_3.setObjectName("pushButton_3")
        self.pushButton_4 = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton_4.setGeometry(QtCore.QRect(340, 170, 441, 41))
        self.pushButton_4.setObjectName("pushButton_4")
        self.pushButton_5 = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton_5.setGeometry(QtCore.QRect(340, 250, 441, 41))
        self.pushButton_5.setObjectName("pushButton_5")
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 800, 18))
        self.menubar.setObjectName("menubar")
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.pushButton_2.setText(_translate("MainWindow", "Airway: 1mm"))
        self.pushButton_3.setText(_translate("MainWindow", "Airway:10 mm"))
        self.pushButton_4.setText(_translate("MainWindow", "Try to breathe independently: on"))
        self.pushButton_5.setText(_translate("MainWindow", "Try to breathe independently: off"))


# мониторит серийный порт
class MyThread(QtCore.QThread): 
    mySignal = QtCore.pyqtSignal(int)
    
    def __init__(self, *args, **kwargs):
        super().__init__()
        self.val = 0

    def run(self):
        while True:
            self.val += 1                     # Получаем определённые данные
            self.mySignal.emit(self.val)      # Передаем данные для отображения
            QtCore.QThread.msleep(1000)       
       
        
class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow):        
    def __init__(self):
        super(MainWindow, self).__init__()
        
        self.setupUi(self)
    
        self.thread = MyThread()
        self.thread.mySignal.connect(self.show_result_func)
        self.thread.start()

    def show_result_func(self, val):
        self.textEdit.append(str(val))


if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    w = MainWindow()
    w.show()
    sys.exit(app.exec_())