Chosen Solution
iPhone 6s Plus overheats, I now have an 8 so not using the 6 but thinking about rebuilding what part would I be best to replace to stop the overheating and improve the performance of this model.
@tweetylove start by replacing the battery, then re-evaluate. If nothing else it gives you a know-well starting point. iPhone 6s Plus Battery Replacement
Completely wipe it out. Settings > general > reset > erase all content and settings. Some applications could cause phones overheating. For example gps requiring apps and iCloud when copying all photos from your old account
