Remember to ALWAYS BACK UP YOUR DATABASE before running any SQL commands.

Check the old key

SELECT * FROM `wp_postmeta` WHERE meta_key LIKE '%old_field_name%'

Take note of the field key “field_” and a random string. You can also check this in ACF Field Group admin page.

Update postmeta

UPDATE `wp_postmeta` as m 
JOIN `wp_posts` as p 
ON m.post_id = p.ID SET m.meta_key = 'new_field_name' 
WHERE m.meta_key = 'old_field_name' 
AND p.post_type = 'my_post_type'

Update ACF field keys

Make sure to have the correct field key (field_…)!

UPDATE `wp_postmeta` as m 
JOIN `wp_posts` as p 
ON m.post_id = p.ID 
SET m.meta_key = '_new_field_name' 
WHERE m.meta_value = 'field_5af0d933478b4' 
AND p.post_type = 'my_post_type'

Now you can rename the field to ‘new_field_name’ in ACF admin.