How to downgrade from iOS 26 to iOS 18.5?

There is no supported downgrade method and there never has been for any iOS update.

When did you update? You should expect battery drain, lagging, and even extra heat generation for 3-4 days after an update for file indexing and photo library optimization for Object Detection algorithms. If you have a large photo library, give it an extra day or two. Leave your phone plugged in all night, every night.

About Apple software updates - Apple Support

If you still experience trouble after that time, then perform a Force Restart:

• Press and quickly release volume up
• Press and quickly release volume down
• Press and HOLD the side button until the Apple logo appears. It could take up to 15 seconds before you see the Apple logo and you must ignore anything else on the screen during that time. If you do not see the Apple logo while holding the side button, then try again as it may take a couple of attempts to get the speed and sequence of the key presses correct.

Downgrading your iOS isn't supported.

After an update it's common for your iPhone to drain more battery as it has to complete the temporary post-update processes including indexing data and files for search, downloading new assets, and updating apps. This doesn't last forever and typically takes a few days to a week.

Regarding the lag, perform a Forced Restart.

• Press and quickly release the Volume Up Button
• Press and quickly release the Volume Down Button
• Press and hold the Side Button until you see the Apple Logo (Past the Power Off Slider, which can take up to 30 sec)

Sorry, downgrading the operating system is not supported.

Please read the following article about software updates. It explains why your phone may be lagging and getting warmer than usual after an update.

About Apple software updates - Apple Support