I have a LG G Stylo (Canadain) modle number LG-H636

I have a LG G Stylo (Canadain) modle number LG-H636
i rooted mt phone and after a day i unrooted it . Now when i try to update my phone it wont update cz it say its suspected of rooting.
How can i flash the stock rom for this phone ? and where can i download the stock rom for this model ?like how can i get it back to normal and then update ? plz help