Как вычесть средний балл в PyQt?

Не изменяйте код, сгенерированный Qt Designer. Создайте другой класс, который наследуется от соответствующего виджета, и используйте созданный класс для его заполнения.

from PyQt5 import QtCore, QtGui, QtWidgets

class Ui_Dialog(object):
    def setupUi(self, Dialog):
        Dialog.setObjectName("Dialog")
        Dialog.resize(412, 239)
        self.label = QtWidgets.QLabel(Dialog)
        self.label.setGeometry(QtCore.QRect(100, 10, 211, 31))
        font = QtGui.QFont()
        font.setFamily("Montserrat")
        font.setPointSize(20)
        font.setItalic(True)
        self.label.setFont(font)
        self.label.setObjectName("label")
        self.one = QtWidgets.QPushButton(Dialog)
        self.one.setGeometry(QtCore.QRect(20, 150, 61, 41))
        font = QtGui.QFont()
        font.setFamily("Montserrat SemiBold")
        font.setPointSize(14)
        font.setBold(True)
        font.setUnderline(False)
        font.setWeight(75)
        self.one.setFont(font)
        self.one.setStyleSheet("background-color: rgb(255, 170, 0);")
        self.one.setObjectName("one")
        self.two = QtWidgets.QPushButton(Dialog)
        self.two.setGeometry(QtCore.QRect(100, 150, 61, 41))
        font = QtGui.QFont()
        font.setFamily("Montserrat SemiBold")
        font.setPointSize(14)
        font.setUnderline(False)
        self.two.setFont(font)
        self.two.setStyleSheet("background-color: rgb(255, 170, 0);")
        self.two.setObjectName("two")
        self.three = QtWidgets.QPushButton(Dialog)
        self.three.setGeometry(QtCore.QRect(180, 150, 61, 41))
        font = QtGui.QFont()
        font.setFamily("Montserrat SemiBold")
        font.setPointSize(14)
        font.setUnderline(False)
        self.three.setFont(font)
        self.three.setStyleSheet("background-color: rgb(255, 170, 0);")
        self.three.setObjectName("three")
        self.four = QtWidgets.QPushButton(Dialog)
        self.four.setGeometry(QtCore.QRect(260, 150, 61, 41))
        font = QtGui.QFont()
        font.setFamily("Montserrat SemiBold")
        font.setPointSize(14)
        font.setUnderline(False)
        self.four.setFont(font)
        self.four.setStyleSheet("background-color: rgb(255, 170, 0);")
        self.four.setObjectName("four")
        self.five = QtWidgets.QPushButton(Dialog)
        self.five.setGeometry(QtCore.QRect(340, 150, 61, 41))
        font = QtGui.QFont()
        font.setFamily("Montserrat SemiBold")
        font.setPointSize(14)
        font.setBold(True)
        font.setUnderline(False)
        font.setWeight(75)
        self.five.setFont(font)
        self.five.setStyleSheet("background-color: rgb(255, 170, 0);")
        self.five.setObjectName("five")
        self.Marks = QtWidgets.QLabel(Dialog)
        self.Marks.setGeometry(QtCore.QRect(10, 50, 391, 31))
        font = QtGui.QFont()
        font.setFamily("Montserrat Medium")
        font.setPointSize(17)
        font.setBold(False)
        font.setItalic(False)
        font.setUnderline(False)
        font.setWeight(50)
        font.setStrikeOut(False)
        font.setKerning(True)
        self.Marks.setFont(font)
        self.Marks.setText("")
        self.Marks.setAlignment(QtCore.Qt.AlignCenter)
        self.Marks.setObjectName("Marks")
        self.Result = QtWidgets.QLabel(Dialog)
        self.Result.setGeometry(QtCore.QRect(140, 100, 131, 31))
        font = QtGui.QFont()
        font.setFamily("Montserrat Medium")
        font.setPointSize(21)
        self.Result.setFont(font)
        self.Result.setMouseTracking(False)
        self.Result.setLayoutDirection(QtCore.Qt.LeftToRight)
        self.Result.setText("")
        self.Result.setAlignment(QtCore.Qt.AlignCenter)
        self.Result.setObjectName("Result")
        self.Apply = QtWidgets.QPushButton(Dialog)
        self.Apply.setGeometry(QtCore.QRect(130, 200, 75, 23))
        font = QtGui.QFont()
        font.setFamily("Montserrat")
        font.setPointSize(10)
        self.Apply.setFont(font)
        self.Apply.setObjectName("Apply")
        self.Clear = QtWidgets.QPushButton(Dialog)
        self.Clear.setGeometry(QtCore.QRect(220, 200, 75, 23))
        font = QtGui.QFont()
        font.setFamily("Montserrat")
        font.setPointSize(10)
        self.Clear.setFont(font)
        self.Clear.setObjectName("Clear")

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

    def retranslateUi(self, Dialog):
        _translate = QtCore.QCoreApplication.translate
        Dialog.setWindowTitle(_translate("Dialog", "Dialog"))
        self.label.setText(_translate("Dialog", "Средний балл"))
        self.one.setText(_translate("Dialog", "1"))
        self.two.setText(_translate("Dialog", "2"))
        self.three.setText(_translate("Dialog", "3"))
        self.four.setText(_translate("Dialog", "4"))
        self.five.setText(_translate("Dialog", "5"))
        self.Apply.setText(_translate("Dialog", "Принять"))
        self.Clear.setText(_translate("Dialog", "Очистить"))

        
class Dialog(QtWidgets.QDialog, Ui_Dialog):
    def __init__(self):
        super().__init__()
        self.setupUi(self)
        
        self.marks = []

        self.add_functions()
        
    def add_functions(self):
        self.one.clicked.connect(lambda: self.write_number(self.one.text()))
        self.two.clicked.connect(lambda: self.write_number(self.two.text()))
        self.three.clicked.connect(lambda: self.write_number(self.three.text()))
        self.four.clicked.connect(lambda: self.write_number(self.four.text()))
        self.five.clicked.connect(lambda: self.write_number(self.five.text()))
        self.Apply.clicked.connect(lambda: self.results())
        self.Clear.clicked.connect(lambda: self.clear())

    def write_number(self, number):
        self.marks.append(int(number))
        self.Marks.setText(self.Marks.text() + number)

    def results(self): #, marks):
#        self.marks = []
        if not self.marks:
            return
        res = sum(self.marks) / len(self.marks)
        self.Result.setText(str(res))
        self.marks = []

    def clear(self):
        self.Marks.setText("")
        self.Result.setText("")
        
        

if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    w = Dialog()
#    ui = Ui_Dialog()
#    ui.setupUi(Dialog)
    w.show()
    sys.exit(app.exec_())