Craft 3 plugin - how to update entry with field of Entries type
Ok this was easy.  Tried it at first but didn't work and was successful after another go.  All it needed was a array of Entry IDs.

Working code:

$entry = Entry::find()-&gt;id($entryId)-&gt;one();
$entry-&gt;textField = 20; // works
$entry-&gt;entriesField = [$relatedEntryId1, $relatedEntryId2]; // works
Craft::$app-&gt;elements-&gt;saveElement($entry);