Thank you! It works perfectly. I do find the update_field() documentation for repeater fields confusing…
// save a repeater field value
$field_key = "field_12345678";
$value = array(
array(
"sub_field_1" => "Foo",
"sub_field_2" => "Bar"
)
);
update_field( $field_key, $value, $post_id );