Hi @mikey2004
In this case, you can grab individual field values from the $_POST data like so:
$field_value_1 = $_POST['acf']['field_xxxxxxxxxxx'];
In this example, we are making use of field key to target a certain field and you can then add a new row of data to an existing repeater by making use of the following function: https://www.advancedcustomfields.com/resources/add_row/
I recommend using field keys in the above function since this allows ACF to correctly find the field if no existing value has been saved.
Thank you.