First off, don't inject variables directly into a query like that - use prepared statements. Secondly, if it's not too late already I recommend switching to the PDO database interface - it's much easier to work with and reason about.
Now, as to your actual question, set the value to null.
$qry = "
UPDATE login_users
SET otp_reset = 0
,otp_expiry = :exp
WHERE username_email = :email
";
$sql = $pdoObject->prepare($qry);
$result = $sql->execute($qry, [
'exp' => null,
'email' => $myusername
]);
Admittedly, I've been in the land of Laravel for about 5 years now and am not entirely sure of my syntax above, so take it with a grain of salt...