I just want to change heading from invoice to tax invoice

[Patch]

See

Tax Invoicing format in UAE

Custom themes is not a solution. Legal requirements are not meant to be solved by custom themes. As far as I can see, Manager complies with UAE requirements. Tax invoice title should be mentioned on all invoices issues This can be achieved by having Custom Title option checked on invoice. Tax Registration Number (TRN) to be displayed This can be mentioned as part of your business address under Settings → Business Details Price displayed should be inclusive of VAT This can be done by…

in particular, Custom Title option checked on invoice
For which see
Sales Invoices: Create sales invoices