Firstly, you are probably aware that altering a file in the plugin is not a good idea. When ACF is updated your changes will be overwritten and need to be made again and again for every update. The best way to make changes is to include a separate custom javascript file that extends the functionality provided by ACF.
That being said, the minified version of the script is manually created from the unminified version whenever there is an update. Which version is loaded is dependent on the value of the SCRIPT_DEBUG constant. The unminified version is loaded when this constant is set to true. https://codex.wordpress.org/Debugging_in_WordPress#SCRIPT_DEBUG
