Check out http://wordpress.org/plugins/advanced-custom-fields/developers/ – you might want to go with 3.5.8, but if that doesn’t work try an earlier one. You will need to replace your existing
/wp-content/plugins/advanced-custom-fields/ folder contents with this new plugin. Alternatively, you might be able to delete the plugin from your site’s plugins page and then upload a new version from your hard drive.
BACK UP YOUR SITE FIRST, both database and files! Lots of things can go wrong here – you don’t want to be caught off-guard. There are lots of backup plugins out there, and your host might offer some sort of backup or restore service as well. Chances are that you will also have to restore your database to a pre-update state as well, as I am fairly sure that there are database chances between ACF v3 and v4. If you don’t have access to your pre-update database, uninstalling ACF from the Plugins page and then re-installing V3 might be your best chance.
Since you’re working with ACF here, it would be worthwhile to export your field definitions either into PHP or XML, just for safe keeping… you might be able to use these definitions to reset your site if anything goes wrong.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users
Helping others is a great way to earn karma, gain badges and help ACF development!