Q: iPhone 6 screen

Hello Amy,

Welcome to Apple Support Communities.

The information in the article below should answer your question about why your iPhone 6 heats up and it should address any related concerns that you may have about operating temperatures of your device. 

Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures - Apple Support

When using the device or charging the battery, it is normal for it to get warm. The exterior of the device functions as a cooling surface that transfers heat from inside the device to the cooler air outside.

Ciao.