In odoo version 8
go to Settings menu > Technical > Reports > Reports
In the search type INVOICES and press ENTER
click on the record
click on purple text "Search associated QWeb views"
click on report_invoice_document
click EDIT button
under Architecture replace <span t-if="o.type == 'out_invoice' and (o.state == 'open' or o.state == 'paid')">Invoice</span> with <span t-if="o.type == 'out_invoice' and (o.state == 'open' or o.state == 'paid')">Tax Invoice</span>

Save
Just to be safe, Logout, restart your server and delete your browser cache
The next invoices printouts you generate will be labelled Tax Invoices. Old ones will not show remain showing Invoice even if you reprint after this change

Hi Thanks, it helped, in odoo 14 it is nearly the same;

with developer mode on

go to Settings menu > Technical > Reporting > Reports
In the search type INVOICES and press ENTER
click on the Invoices without Payment record
click on " > QWeb views"
click on report_invoice_document
click EDIT button
under Architecture replace 

Invoice with
Tax Invoice

If you want to change words, you have to modify the translation (Settigns -> Application terms -> Translated Terms.

Search for the terms you want to change.

Important: when you do an upgrade/installation of a language, do not tick the option: 'Over write existing terms'!