Noob question – how to change 'Default sorting' to 'Default Sorting'

You can use the .po file for overriding the texts.

Like your site language is set to English (United States). So your file name would be woocommerce-en_US.po. Open that, edit whatever text you want to change, save and place that to wp-content/languages/plugins/

For more details on translation, you should read this doc https://docs.woothemes.com/document/woocommerce-localization/

k thx! will try it