Q: the center button on my iPhone 6 heats up even when its not charging.  I've been using it for a day only.

Greetings ankitapant,

Welcome to the Apple Support Communities!

I understand that your iPhone is getting warm and you are concerned about it. It is, however, normal for an iPhone to get warm while charging or while in use. For more information about operating temperatures, please review the attached article. 

https://support.apple.com/en-us/HT201678

Cheers,

Joe