By Default iPhone 6 and iPhone 6 plus are record videos at 30 fps. you need to make few changes to enable 60 fps feature. Follow these simple instruction given below.
1. Open Settings page
2. In settings page find “Photos & Camera” you will find some options.
3. at the bottom turn on “Record Video at 60 FPS”
After the 60 fps feature is Enabled, record videos as you normally do .in playback you will see huge different in Video Quality and your video file Size will increase.