There is no way to update the subscriber through WIX then; it will also update on Convertkit.
Convertkit only offers an option to integrate with WIX with this guide HERE10.
What happens is once a subscriber sign up with WIX, the data of the subscriber will be delivered to Convertkit, and if you plan to update that subscriber the update needs to be done on Convertkit's end.
I hope it helps. If you're still having issues, you can always reach out to firstname.lastname@example.org.