Amend Invoice to Tax Invoice

i would like to know how do i amend invoice to tax invoice as i cant find the html code.

If you select a tax code that is known to require this change, it happens automatically. See this Guide: Otherwise, see this Guide:

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if your using the latest version of Manager, then the Indian tax codes are in-built.
you will have to set your tax codes under settings.
then if you select one of these tax codes while making a sales invoice, the title will automatically change to Tax Invoice.