to update to next value in textbox as it is used for invoice system

Although you can, I wouldn't do it.

Doing it that way relies on the user not modifying the value and inserting a non-numeric value.

The easiest way is to keep the current invoice number as a class level integer variable, and increment it when necessary and re-write the contents of the text box.

private int invoiceNumber = 1000;
...
private void NextInvoice()
   {
   invoiceNumber++;
   TextBoxForInvoiceNumber.Text = invoiceNumber.ToString("000000");
   }

But I would use either a Label instead of a TextBox, or make the TextBox ReadOnly property true.

If you must use the TextBox content, you have to do:

private void NextInvoice()
   {
   int invoiceNumber = int.Parse(TextBoxForInvoiceNumber.Text);
   invoiceNumber++;
   TextBoxForInvoiceNumber.Text = invoiceNumber.ToString("000000");
   }

And remember to add error trapping and so forth to prevent your app falling over when the user makes a small mistake...