Page.ClientScript.RegisterStartupScript(Page.GetType(), null, "window.open('bill_invoice.aspx?Number=" + txt_billNo.Text + "', '_blank')", true);
This is a new tab for the bill invoice.
Page.ClientScript.RegisterStartupScript(Page.GetType(), null, "window.open('customerRegistration.aspx', '_self')", true);
This is the old page that needs to be refreshed.
The new page opens in a new tab, but the old page cannot be refreshed or opened in the same tab.