.Net code for Changing the values in the Label according to the buttons Next, Previous , Last and First. The value of Label must be between 1 to 20.

Just use the counter.
By clicking on the "Next" increases by one.
By clicking on the "back" button, reduces it to one.

Each time you press a button after changing the value of counter, you should assigning his value to the property "text" of the button

For example:

private int counter;

private void btnNext_Click(object sender, EventArgs e)
{
           counter++;
           if(counter < 21 && counter > 0 )
           {
             myLabel.Text = counter.ToStreing();
           }
}

private void btnBack_Click(object sender, EventArgs e)
{
         counter--;
         if(counter < 21 && counter > 0 )
         {
             myLabel.Text = counter.ToStreing();
         }
}

Of course you can use a function,for example SetLabel, and call it from both places .....

Regards,
Alex.