ACF: How to programmatically Update Flexible Content Field?

I want to be able to create sub fields inside a flexible content field.

My setting is this one:

menu_item_field is a flexible content field that contains:

This is the structure that results of the above setting:

Group One
Item 1 title
Item 1 description
Item 1 img 1
Item 1 img 2
Item 2 title
Item 2 description
Item 1 img 1
Item 1 img 2

So far I’ve been able to insert only the Group field and I wanted to know if it’s possible to do more complex stuff.

I did it using:

The above code returned only the main group titles:

I know there’s update_sub_row()

but I haven’t been able to successfully use it.

My main goal is to be able to edit or create new fields using a custom UI in the frontend through AJAX.

I prefer this instead of acf_form().

So, my question, is it possible to achieve what I need and what would be the proper function with the above settings.

Thanks.