If you carefully check your flow, you will see that the second 'incorrect username or password' message box is outside any conditional branch (if)! That's the reason it always shows.
So adding the correct else will fix it, however you should not have that messages twice, but instead as about username and password in a single if, like this:
if ((textBox1.Text == "Quinn") && (textBox2.Text == "Qmanqtip12"))
{
System.Threading.Thread.Sleep(4000);
MessageBox.Show("Welcome To AL7", "Login Succesfull",
MessageBoxButtons.OK, MessageBoxIcon.Information);
new Form2().Show();
this.Hide();
}
else
{
MessageBox.Show("Usename or Password Incorrect", "Error",
MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
}