How to fix my iPhone battery drain after update?

Thuugh you did not mention which iOS update I guess probably you are referring to iOS 17.5.1.

It surely is not after iOS 17.5.1 because...

I conducted a thorough search of this forum and uncovered numerous posts detailing similar issues across various iOS updates. This suggests that iOS updates may not consistently be the root cause. Below are a handful of examples extracted from the search results:

The links I've found suggest that the reason for battery drain may not be solely due to iOS updates, but rather, it could be influenced by how an iPhone is utilized.

iOS 16

• Upgrade to IOS 16 causing rapid battery draining
• iOS 16.6, iPhone 13, battery drain
• iOS 16 draining battery on iPhone 13

iOS 15

• battery draining after ios 15
• Ip 12 pro max ios 15 drain battery too fast
• Apps draining battery - (iPhone 8, IOS15.1)

iOS 14

My battery drains so fast since upgraded to iOS14, mine is 7 Plus 256GB

IOS14 Health app battery drain is significant

Battery is draining fast after iOS 14

iOS 13

iOS 13 draining my battery

Re: iOS 13.1.2 Battery Drain on iPhone

Re: iOS 13.2.3 iPhone Update – Severe Battery Drain

iOS 12

iOS 12 iPhone 6s battery drain

iPhone SE battery draining fast with iOS12

iOS 12 battery drain too painful

It's not always the iOS update's fault!  Many things can cause phone problems.  If you tell people you think it's an iOS update, it might make it harder to fix or find a solution for the real issue.

Remember, the right question often leads to the solution! Please look in the right direction.

iOS updates aim to enhance user experience, boost security, and fix bugs rather than disrupt core features. The majority of the issues after updates might stem from underlying, unmentioned factors. Updates won't typically alter personal data but may refine settings or features. Backup before major updates for safety. Review Apple's update details and backup advice for a smooth process.

Kindly consider checking your settings and addressing any device issues before attributing the issue to external factors. This could be a pure coincidence that the iOS update affected the iPhone functions.

Before initiating a standard iOS update, it's advisable to create a backup. Have you completed this step? If yes, proceed with the restoration from the backup.

See what each update does: About iOS 17 Updates, About iOS 16 Updates, About iOS 15 Updates, About iOS 14 Updates, About iOS 13 Updates, About iOS 12 Updates, and so on...