Problems updating nested ACF field groups

I have a frontend form for creating custom posts that accepts data for all ACF associated with that post type. On submission of the form, I am successfully able to create the new post along with pushing normal ACF fields values.

The problem is with the ‘Group’ ACF field called ‘Working Hours’ which further has 7 Groups inside it, each named after every day of the week: ‘Monday’, ‘Tuesday’… And each day group contains 2 text fields: am, pm, and a third checkbox called ‘closed‘ which the member will select if that is a weekly off. So it is like this:

#Working Hours( Group )#

——-##Monday ( Group )##

———— ###am (text)###
———— ###pm (text)###
———— ###closed (checkbox)###

I am getting an error undefined index: sub_fields...
And this is my code that I am trying to update the fields:

// Grabbing the Working Hours Group field
$hours = get_field( 'field_61f453c010074', $post_id );

foreach ( $hours['sub_fields'] as $key => $value ) { // error in sub_fields in this line
    foreach ( $value['sub_fields'] as $timing ) {
        $timing[0] = $_POST['field_61f4573589282'];// am
        $timing[1] = $_POST['field_61f4574589283'];// pm
        $timing[2] = $_POST['field_61f7a28ffbcef'];// 'Closed' checkbox
    }
}

update_field( 'field_61f453c010074', $value, $post_id ); // error here too value is undefined

This is how the Working Hours Group field is printed. Here, I have posted the data returned for the parent group, as well as taken only the first child for sample i.e. Monday:

Array
(
[ID] => 48
[key] => field_61f453c010074
[label] => Hours
[name] => hours
[prefix] => acf
[type] => group
[value] => 
[menu_order] => 9
[instructions] => 
[required] => 0
[id] => 
[class] => 
[conditional_logic] => 0
[parent] => 36
[wrapper] => Array
(
[width] => 
[class] => 
[id] => 
)
[layout] => block
[sub_fields] => Array
(
[0] => Array
(
[ID] => 49
[key] => field_61f4570e89281
[label] => Monday
[name] => monday
[prefix] => acf
[type] => group
[value] => 
[menu_order] => 0
[instructions] => 
[required] => 0
[id] => 
[class] => 
[conditional_logic] => 0
[parent] => 48
[wrapper] => Array
(
[width] => 
[class] => day
[id] => 
)
[layout] => block
[sub_fields] => Array
(
[0] => Array
(
[ID] => 50
[key] => field_61f4573589282
[label] => am
[name] => am
[prefix] => acf
[type] => number
[value] => 
[menu_order] => 0
[instructions] => Only enter numbers
[required] => 0
[id] => 
[class] => 
[conditional_logic] => 0
[parent] => 49
[wrapper] => Array
(
[width] => 
[class] => am
[id] => 
)
[default_value] => 
[placeholder] => 
[prepend] => 
[append] => 
[min] => 
[max] => 
[step] => 
[_name] => am
[_valid] => 1
)
[1] => Array
(
[ID] => 51
[key] => field_61f4574589283
[label] => pm
[name] => pm
[prefix] => acf
[type] => number
[value] => 
[menu_order] => 1
[instructions] => Only enter numbers
[required] => 0
[id] => 
[class] => 
[conditional_logic] => 0
[parent] => 49
[wrapper] => Array
(
[width] => 
[class] => pm
[id] => 
)
[default_value] => 
[placeholder] => 
[prepend] => 
[append] => 
[min] => 
[max] => 
[step] => 
[_name] => pm
[_valid] => 1
)
[2] => Array
(
[ID] => 52
[key] => field_61f7a28ffbcef
[label] => closed
[name] => closed
[prefix] => acf
[type] => checkbox
[value] => 
[menu_order] => 2
[instructions] => 
[required] => 0
[id] => 
[class] => 
[conditional_logic] => 0
[parent] => 49
[wrapper] => Array
(
[width] => 
[class] => closed
[id] => 
)
[choices] => Array
(
[Yes] => Yes
)
[allow_custom] => 0
[default_value] => Array
(
)
[layout] => vertical
[toggle] => 0
[return_format] => value
[save_custom] => 0
[_name] => closed
[_valid] => 1
)
)
[_name] => monday
[_valid] => 1
)