Retrieve the Query Builder after filtering the results

To retrieve the Query Builder after filtering the results: you can use ->toQuery().

The method internally use the first model of the collection and a whereKey comparison on the Collection models.

 1// Retrieve all logged_in users
 2$loggedInUsers = User::where('logged_in', true)->get();
 3 
 4// Filter them using a Collection method or php filtering
 5$nthUsers = $loggedInUsers->nth(3);
 6 
 7// You can't do this on the collection
 8$nthUsers->update(/* ... */);
 9 
10// But you can retrieve the Builder using ->toQuery()
11if ($nthUsers->isNotEmpty()) {
12    $nthUsers->toQuery()->update(/* ... */);
13}

Tip given by @RBilloir