Hello there, bms-lou.
It sounds like you are trying to record video at 60 fps with your new iPhone 6 Plus. The following information from the online iPhone User Guide provides the answer on how to do just that:
Take photos and videos - iPhone
Shoot some video. Choose Video, then tap the Record Video button or press either volume button to start and stop recording. Video records at 30 fps (frames per second). With iPhone 6 or iPhone 6 Plus, you can switch it to 60 fps in Settings > Photos & Camera.
Snap a still while recording: (iPhone 5 or later) Tap the white Take Picture button in the lower-left corner.
Take it slow: (iPhone 5s or later) Choose Slo-Mo to shoot slow motion video. You can set which section to play back in slow motion when you edit the video. On iPhone 6 and iPhone 6 Plus, you can tap the bottom-right corner of the screen to switch between 120 fps and 240 fps.
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Kind Regards.