void MyButton::on_pushButton_clicked()
{
    ui->lineEdit->setText("Hi");
}

I see a slot which I believe will be called correctly.

But I see no widget named lineEdit (or for that matter any QLineEdit) anywhere in the ui_...h file. I would therefore conclude that your statement ui->lineEdit->setText("Hi"); will not compile.

/********************************************************************************
** Form generated from reading UI file 'mybutton.ui'
**
** Created by: Qt User Interface Compiler version 6.4.0
**
** WARNING! All changes made in this file will be lost when recompiling UI file!
********************************************************************************/

#ifndef UI_MYBUTTON_H
#define UI_MYBUTTON_H

#include <QtCore/QVariant>
#include <QtWidgets/QApplication>
#include <QtWidgets/QHBoxLayout>
#include <QtWidgets/QMainWindow>
#include <QtWidgets/QMenuBar>
#include <QtWidgets/QPushButton>
#include <QtWidgets/QStatusBar>
#include <QtWidgets/QTextEdit>
#include <QtWidgets/QWidget>

QT_BEGIN_NAMESPACE

class Ui_MyButton
{
public:
    QWidget *centralwidget;
    QWidget *widget;
    QHBoxLayout *horizontalLayout;
    QTextEdit *textEdit;
    QPushButton *pushButton;
    QMenuBar *menubar;
    QStatusBar *statusbar;

    void setupUi(QMainWindow *MyButton)
    {
        if (MyButton->objectName().isEmpty())
            MyButton->setObjectName("MyButton");
        MyButton->resize(800, 600);
        centralwidget = new QWidget(MyButton);
        centralwidget->setObjectName("centralwidget");
        widget = new QWidget(centralwidget);
        widget->setObjectName("widget");
        widget->setGeometry(QRect(50, 5, 222, 211));
        horizontalLayout = new QHBoxLayout(widget);
        horizontalLayout->setObjectName("horizontalLayout");
        horizontalLayout->setContentsMargins(0, 0, 0, 0);
        textEdit = new QTextEdit(widget);
        textEdit->setObjectName("textEdit");

        horizontalLayout->addWidget(textEdit);

        pushButton = new QPushButton(widget);
        pushButton->setObjectName("pushButton");

        horizontalLayout->addWidget(pushButton);

        MyButton->setCentralWidget(centralwidget);
        menubar = new QMenuBar(MyButton);
        menubar->setObjectName("menubar");
        menubar->setGeometry(QRect(0, 0, 800, 22));
        MyButton->setMenuBar(menubar);
        statusbar = new QStatusBar(MyButton);
        statusbar->setObjectName("statusbar");
        MyButton->setStatusBar(statusbar);

        retranslateUi(MyButton);

        QMetaObject::connectSlotsByName(MyButton);
    } // setupUi

    void retranslateUi(QMainWindow *MyButton)
    {
        MyButton->setWindowTitle(QCoreApplication::translate("MyButton", "MyButton", nullptr));
        pushButton->setText(QCoreApplication::translate("MyButton", "PushButton", nullptr));
    } // retranslateUi

};

namespace Ui {
    class MyButton: public Ui_MyButton {};
} // namespace Ui

QT_END_NAMESPACE

#endif // UI_MYBUTTON_H

What do you have in mind?

#ifndef MYBUTTON_H
#define MYBUTTON_H

#include <QMainWindow>

QT_BEGIN_NAMESPACE
namespace Ui { class MyButton; }
QT_END_NAMESPACE

class MyButton : public QMainWindow
{
    Q_OBJECT

public:
    MyButton(QWidget *parent = nullptr);
    ~MyButton();

private slots:
    void on_pushButton_clicked();

private:
    Ui::MyButton *ui;
};
#endif // MYBUTTON_H

Here is a snippet of my code to illustrate: the cpp file:

#include "mybutton.h"
#include "./ui_mybutton.h"

MyButton::MyButton(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MyButton)
{
    ui->setupUi(this);
}

MyButton::~MyButton()
{
    delete ui;
}


void MyButton::on_pushButton_clicked()
{
    ui->lineEdit->setText("Hi");
}

I had a hard time trying to have an interaction to find a solution to display some text. See you soon!

Please show your code.

That said, beside YouTube, the Qt documentation is full of examples to get you started. You might want to dig in it a bit.