If you downloaded you have a archived zip file. Inside the archive will be:

1. Copy g_invoice, to “whmcs_dir/modules/addons/”.

2. You should change chmod permissions license.key’s in g_invoice folder. If you dont change this file’s permission, you cant use this module. You should change permission to 766.

3. Create a new folder named “overrides” in “whmcs_dir/lang/” (If it’s exist go to next instruction.)
   

4. Create a new english.php in the folder you opened and write the following codes into it. (If it’s exist write in this file.)

if (!defined("WHMCS")) die("This file cannot be accessed directly");

$_LANG['locale'] = "en_GB";
$_LANG['g_invoices']["extend_button"] = "Extend";
$_LANG['g_invoices']["extend_description"] = "How many months do you want extend it?";