I need an upgrade from ACF v2.1.3 to the most recent one. Where and how to start? My site has 6 fields and 4 different field types (text, textarea, image and file). It’s not probably a trivial process but I need to start from somewhere…
Going from version 2 through the current version I would do a little at a time.
Are we talking about the free version or pro version?
Are you sure that you only have the types of fields that you mentioned?
If you are sure about the field types I would still recommend an incremented update, for example if you go here https://wordpress.org/plugins/advanced-custom-fields/advanced/ you can download older versions. To go from 2.1.3 to the current version I would update by uploading the files of newer versions one at a time and then load the admin and hope that any ACF upgrade process that needed to run would happen. I would do this with the following versions in order:
Before starting back up your database. After each update make sure your site is running. It will be a slow, tedious, time consuming project.