Windows Consumer Support > Services & apps > Windows, then Technical support
If your Clouder PRO account is recognized, the "Go PRO" orange button in the top-right corner should disappear.
You can also try to uninstall and re-install the application from the Store. Make sure you are connected to the Windows Store Application with the same Microsoft account that you bought PRO with.
If it still doesn't work, you will have to contact Microsoft Store Support, which manage the in-app purchases.
You need to contact their Answer Desk, here a direct link:
If you tried these options and still need further assistance, please email firstname.lastname@example.org