I’m a little confused by some of your question.
load_value (https://www.advancedcustomfields.com/resources/acf-load_value/) and update_value (https://www.advancedcustomfields.com/resources/acf-update_value/) are hooks that allow filtering, they are not functions to call. You can effect the value of these fields after they are loaded and before they are saved. These are not functions that you can call.
prepare_field is also a hook, this is only used in the admin just before a field is displayed for editing https://www.advancedcustomfields.com/resources/acf-prepare_field/, again, not a function you should be calling directly. Modifying a field value on this hook will not effect the field.
get_field() and update_field() are the most efficient, and really the only way to get or change the value of a field using ACF.
