Why is my iPhone heating up?
It's saying temperature too high
[Re-Titled by Moderator]
iPhone 6s Plus, iOS 15
Question marked as
Either the ambient temperature is too high, or maybe the battery is failing.
An iPhone 6s with the original battery can be old enough to need battery replacement, or you’re headed for a newer (used, refurb) or new iPhone.
Battery health of 80% or lower usually means repair or replacement.
If your iPhone or iPad gets too hot or too cold - Apple Support
iPhone Battery and Performance - Apple Support
iPhone Battery Repair & Replacement - Apple Support