Welcome to the Apple Support Communities. I see from your post that your iPhone 6 is feeling warmer to the touch since you upgraded to iOS 11. I can understand being concerned about this, and I am happy to help.
Please see the following resource, especially this section, for further information:
Your device might get warm
When you use the device or charge the battery, the device might get warm. You might notice that your device feels warmer in these situations:
- When you set up your device the first time
- When you restore from a backup
- When apps reindex or reanalyze data, like Photos tagging for faces, places, or keywords after a software update
These conditions are normal, and your device will return to a regular temperature when complete.
Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures
Have a fantastic day!