In order to modify strings you need to find translation file. For plugins these are usually here.
In case of WooCommerce please take following language files from this folder.
You can edit these by using Poedit. Install Poedit to your computer and open .po file. Translate or modify the string accoringly and then save.
/htdocs/wp-content/languages/woocommerce – this is correct folder where edited language files should be uploaded in case of WooCommerce IF you do not want to lose translations during plugin update.
But, if the string is not available like here and here there may be different ways to fix it.