IOS 11.2.6 causing overheating

Try this basic troubleshooting step: Make a full backup of your device, and reset it to factory settings, setting it up as new. This operation wil install a fresh copy of the latest iOS version, following the instructions provided in this support guide:

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

When done setting it up as a new device, test it for some hours. If everything is working fine, (no overheating. no battery draining), restore your iPhone from your previuosly made backup.

Regards

Giulio

I have an iPhone 7 that is experiencing the same thing since updating yesterday. When in use or charging, it gets very warm, like it's doing something super-intensive, and the battery drains at around 1-2% per minute. When the screen is locked and the phone is idle, the effect is lessened. Before the update, I could regularly get through the whole day without charging and now I'm having to put it on a charger every few hours.

Depends on what you mean by "overheating". How hot is it getting? What are you using it for when that happens? Does it happen while charging?