For those of you still struggling with this problem, I’ve found a super light and easy-to-use plugin that works like charm. It’s called Edit Custom Fields: https://wordpress.org/plugins/edit-custom-fields/. It lists all custom fields DB entries automatically (both, active and old), allowing you to rename or delete them on a per-name basis. No need of MySQL tweaking!
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users