I would give up on updating the row and instead update the entire group field, the only question would be what other fields are in that group. This should work.
// this will hold the value to update the group with
$group_field_value = array();
// initialize row value just in case the loop returns noting
$row = array();
// loop group field and get existing values
if (have_rows('field_q3md98zvnm241', $post_id)) {
while (have_rows('field_q3md98zvnm241', $post_id)) {
// this loop always happens 1 time for a group field
// *********************************************************
// little known fact
// the_row() returns an array holding the row
// with field keys as indexes and unformatted field values
// *********************************************************
$row = the_row();
}
}
// set a new value for the "owner_name" field by field key
$row['field_4v8yw6n2hbpqs'] = 'get old value and insert here';
// update the field
// update the group field
update_field('field_q3md98zvnm241', $row, $post_id);
