**** backup database first, do not attempt to do it without doing so, and i take no responsibility if there is a problem and you didn’t backup.
1. upload ACF PRO
2. de-activate old acf plugin
3. activate ACF PRO
4. a box at the top appears saying to update database (it will change certain things in the database fields etc to clean up the old acf fields to match the new ACF PRO structure)
5. check your site over to see everything is running.
6. delete from FTP the old ACF plugin as it is no longer referenced.