I solved it, I just had to add a counter in the array( value[“here”] ), so that if, lets say, I wanna modify the information of the 1st row, then id put something like this:
$counter = 0;
$value[$counter] = array("fecha" => $fech4,
"saldo" => $sal14,
"inversion_en_el_periodo" => $ielp4,
"interes_causado_en_el_periodo" => $icelp4,
"cantidad_pagada" => $cp4,
"cantidad_reinvertida" => $cr,
"saldo_final" => $sf4,);
update_field( $field_key, $value, $post_id );
$counter=$counter+1;
and then keep looping it around