For Hard Reset Your Battery Charge should be 60% OR 70% Up
Backup your All Contact, Message, photo, videos, music and others Data on pc or online storage.
Remove Your Sim Card.
Goto Your Home
Now Tap On Settings

Select This Option > General
Now Select > Reset
now select > Erase All Content And Settings
Wait Few second Mobile Will be Auto reboot
Method 2 :
1. Turn off Your Device iphone 6
2. Keep Pressing Power Key + Home Key 10 second Release all keys show apple logo.
Warring : hard Reset Will be Erase All OF Your Data. Please Backup Your All Data Before Do This.