CoreCommerce Addon simple question1 user found helpful
concerning the address fields, it's a core user attribute, not part of the e-commerce add-on but part of C5. If you want to change it, you would have to override the core attribute.
Look in concrete/models/attribute/types/address/form.php
That's the file you need to override.
Modify line 20
ss.append('<option value="">Choose State/Province</option>');
It should do the trick