Since you have updated your license to Pro, you will have to have Pro installed and activated on your site since you will not be able to validate an X site using a Pro license and Pro using an X license.
You may get the installable file for Pro here:
It is always recommended to have the child theme installed and active in case you will need to add some customization on your site. You can get a copy of the child theme here:
Once you have Pro and Pro child installed on the site, activate Pro child then you can now validate the site using the license key from your licenses page here:
You can read more about the validation here:
In case you need more information about conversion from X to Pro, you can read more about it here:
Hope this helps.