Hello @chicadita,
The naming for the tax can only be changed in the cart page. Currently to change the naming in the checkout page is not possible but we will definitely make it work in the coming update of the plugin.
Now, as for the renaming of the tax in the cart page, you have to activate the child theme.
To know how to create and activate the child theme please follow below given link:
http://wptravel.io/how-to-create-a-child-theme/
After activating the child theme, in your child theme create the new file “content-cart.php“. After creating the file add below given code in the file “content-cart.php“.
https://pastebin.com/zAUAx88M
Hope this helps.
Regards!!