How to change (re-order) the deltas programmatically in node_presave
You would want to set the field value to an array in the order you want them. Since you are doing this inside hook_node_presave, you don't have to bother saving the node. It will be done for you.

Something like this should work:

$values = $node-&gt;get('field_mv')-&gt;getValue();

... your sorting function ...

$node-&gt;set('field_mv', $values);