When the template is installed, that file would get moved to language/en-GB/en-GB.tpl_protostar.ini
, so that would be the file that actually needs to get updated.
That being said, a better method is to use the language override feature on the admin side. You'll see in that file that the language key is TPL_PROTOSTAR_BACKTOTOP
(what is to the left of the equal sign).
On the admin side, go to Extensions -> Language Manager. Then on the left side of the page, click "Overrides". Hit the new button and add TPL_PROTOSTAR_BACKTOTOP
as the constant and whatever new text you would like there in the "Text" box.
By using the override feature you protect your site from updates if the template's language file is updated in a later version.