You have 2 problems.
1) You’re using a checkbox field. Checkboxes are array fields and the values stored in the database are serialized arrays. You can’t update the values in these fields with a simple query. In fact, I don’t thing that any query directly on the DB will work. I would suggest trying this tool https://interconnectit.com/products/search-and-replace-for-wordpress-databases/
2) You also need to update the field definition. Setting a value to a value that does not exist in the fields “choices” will cause the field to show no value the next time the page is loaded for edit. The field definition is stored as a serialized array in the “post_content” column of the “acf-field” post. If you use the tool I mentioned above, it will replace these as well.
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