from PySide6.QtWidgets import QApplication, QMainWindow
from pyqt5_gui import Ui_MainWindow

class Frm_main(QMainWindow, Ui_MainWindow):
    def __init__(self):
        super().__init__()
        self.setupUi(self)

app = QApplication()
frm_main = Frm_main()
frm_main.lbl_eins.setText("Ein Label")
#self.lbl_eins.setText(QCoreApplication.translate("MainWindow", u"Das Label", None))
frm_main.show()
app.exec()