Hello @raffitireland,
To change the word “Shopping Cart” in the Header to “Basket” you have to activate the child theme .
Child theme references:
https://codex.wordpress.org/Child_Themes
You can also create the child theme using the plugin . Below is the link to one of them.
https://wordpress.org/plugins/wp-child-theme-generator/
After activating the child theme in your child theme functions.php file paste below given code:
https://gist.github.com/anonymous/3844606aec6710a0cbe99ef794832027
And yes you have go through the child theme approach to make any change in the theme as your original modification wont be lost.
Hope this helps.
Best Regards!!