i need GST template for sales invoice
You don’t get a GST template, you go to Settings - Tax Codes and select your country’s tax rate.
This will then cause the GST to appear on the Sales Invoice.
i need to create a template and its not getting updated
You need to provide more information. Getting a template is one thing. Go to the Templates site: https://www.manager.io/templates and copy the code. Then create a template under Settings => Templates, using that code. Updating a template is totally up to you.
There is no default template you can use directly as a GST template. You need to use the latest version of Manager and make some manual settings to get everything GST ready. Follow the below topic for details.