ui->lineEdit_2->setEchoMode(QLineEdit::Normal); ui->lineEdit_2->setEchoMode(QLineEdit::Password);

#include "dialog.h"
#include "ui_dialog.h"
#include<QLineEdit>
Dialog::Dialog(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Dialog)
{
    ui->setupUi(this);
}

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

void Dialog::on_pushButton_3_pressed()
{
    ui->lineEdit_2->setEchoMode(QLineEdit::Normal);
}

void Dialog::on_pushButton_3_released()
{
   ui->lineEdit_2->setEchoMode(QLineEdit::Password);
}