Thank you for your answer that solves part of my problem.
However I have unique values in my acf_form () form with and I want to have a field which allows to insert a unique value and modify a field of the row
if (have_rows('rsvp')) {
while (have_rows('rsvp')) {
the_row();
if (get_sub_field('unique_id') == '5efb792f219a9') {
update_sub_field('confirmation', 'accept');
}
}
}