Everybody I know just installed the new iOS and everybody is texting me the same thing: "My battery is dead by lunch. Did the update break it?"
Usually, no. Your phone is just busy.
The first day or two
After a big update the phone spends a while reorganizing itself in the background: re-indexing your stuff, grabbing new system files, updating apps to match. Apple has said itself that battery drain can run for a day or two afterward. So the phone that's warm in your pocket on a Carlsbad beach walk is working, not dying.
Plug it in, leave it on Wi-Fi overnight, and let it finish. Resist the urge to restart it nine times.
Two cheap checks before you panic
Open the battery screen in Settings and see which app is eating the most. One app sitting at the top for no reason is usually a software glitch, and updating or reinstalling that app is free. It's a lot cheaper than a new battery.
Then feel the phone. Warm while it's charging and reindexing is normal. Hot on the dash, hot in your hand, hot while it's doing nothing at all, that's the one I want to hear about.
Also, don't charge it under a pillow or in direct sun on the patio while it's busy. A phone already working hard doesn't need extra heat on top.
When it's not the update
If it's still draining fast after a few days, look at the phone, not the software. A battery that's a couple of years old and has been baking in a hot car is a much more common culprit than an update. Other signs: the percentage jumps around, the phone shuts off at 30%, or it gets hot just sitting there.
One thing I'd never ignore: a screen that starts lifting or a back that bulges. That's a swollen battery, and it needs a repair, not patience.
So wait out the two days. If the drain is still there after that, I can swap the battery at your door in Carlsbad, Oceanside, Vista, San Marcos or Escondido.