The Phartail system is listed as commercial. This means you will have to buy it in order to access the downloads and help system or any other files that might be bundled with the application.
Because of that, I can't really say what is the exact procedure to change the tax group in the application.
If you have it installed on your PC, then access the built-in Help section to find out about the correct procedure since it's the only available source.
Additionally, reach the developers and ask for their help in this matter: http://softman.in/contact-us.html