Hi Giang, the advice you're getting here is fine, but you should be able to walk into any Optus store and say ''hey, my update didn't work, can you help?'' and they should just have a crack at it there and then without charging you (at least, i always found Optus to be friendly enough, not like Telstra!). The issue is just an update bootloop which is very common, so the average guy in the store should be very familiar with it.
If you want to go the manual fix route, you can check out this tutorial I wrote a while back to flash firmware manually - of course, it's a different device but the steps are identical, you just need to make sure you download the right firmware from Sam Mobile's firmware page. You can also just pull the battery for 30 seconds, reinsert it and try the button combination again for the factory reset. If that still doesn't take you to stock recovery then you'll have to fix the firmware yourself as mentioned above or take the device into a store.
Let us know how you go and good luck!