from PyQt6 import QtWidgets,QtCore
import sys
class Clicker(QtWidgets.QWidget):
    def __init__(self):
        super(Clicker, self).__init__()
        self.window=QtWidgets.QWidget()
        self.window.resize(1000,1200)
        self.setWindowTitle('program')
        self.counter=QtWidgets.QLabel('0')
        self.plus_btn=QtWidgets.QPushButton('+1')
        self.ex_btn=QtWidgets.QPushButton('&Exit')
        self.btn_box=QtWidgets.QHBoxLayout()
        self.btn_box.addWidget(self.plus_btn)
        self.btn_box.addWidget(self.ex_btn)
        self.box=QtWidgets.QVBoxLayout(self.window)
        self.box.addWidget(self.counter)
        self.box.addLayout(self.btn_box)
        self.count=0
        self.init_functions()
    def init_functions(self):
        self.plus_btn.clicked.connect(self.pluser)
        self.ex_btn.clicked.connect(app.quit)
    def pluser(self):
        self.count+=1
        self.counter.setText(f'now {self.count}')
app = QtWidgets.QApplication(sys.argv)
w = Clicker()
w.show()
app.exec()

 import sys
from PyQt6 import QtCore, QtWidgets
class Clicker(QtWidgets.QMainWindow): # <======
    def __init__(self):
        super().__init__()
        self.resize(1000,1200) # <======
        self.widget=QtWidgets.QWidget()
        self.setCentralWidget(self.widget) # <======
        self.setWindowTitle('program')
        self.counter=QtWidgets.QLabel('0')
        self.plus_btn=QtWidgets.QPushButton('+1')
        self.ex_btn=QtWidgets.QPushButton('&Exit')
        self.btn_box=QtWidgets.QHBoxLayout()
        self.btn_box.addWidget(self.plus_btn)
        self.btn_box.addWidget(self.ex_btn)
        self.box=QtWidgets.QVBoxLayout(self.widget)
        self.box.addWidget(self.counter)
        self.box.addLayout(self.btn_box)
        self.count=0
        self.init_functions()
    def init_functions(self):
        self.plus_btn.clicked.connect(self.pluser)
        self.ex_btn.clicked.connect(self.close)
    def pluser(self):
        self.count+=1
        self.counter.setText(f'now {self.count}')
app = QtWidgets.QApplication(sys.argv)
w = Clicker()
w.show()
app.exec()