Your iPhone is normally such a reliable device, you can easily forget that it's basically a complex portable computer. The telephone side rarely suffers problems, apart perhaps from the occasional signal blackspot.
If you're having synching problems, check whether your iPhone 6S or iPhone 6S Plus is set to sync only over Wi-Fi rather than 3G/4G too, and whether iCloud or iTunes is selected for backing up – it will back up using one, but not both.
Apart from Wi-Fi, problems are most often down to apps misbehaving. If your iPhone is acting up, try the following steps, in this order. Don't go past step 3 unless turning your iPhone off and on again doesn't resolve the problem or you can't turn it off in the first place.
1. Quit an app
If you run into problems with an app, the first step is to quit it. Double-press the Home button to enter Multitasking View, then swipe sideways until you locate the app in question.
Now swipe its thumbnail preview upwards to dismiss it. Press the Home button once to return to the Home Screen. You can now open the app again if you wish.
On rare occasions, you may not be able to quit an app using the multitasking display, or possibly even exit the app at all. In this case you'll have to force-quit. Hold the Sleep/Wake button for 3-5 seconds until the power-off slider appears, then press and hold the Home button for another 3-5 seconds.
You'll be bounced back to the Home Screen and the app will have quit.
3. Turn it off and on again
If force-quitting an app doesn't fix your problem, try turning your iPhone off and on again. Hold down the Sleep/Wake button again, until the power slider appears. This time, slide it across to power off. Wait a few seconds, then press the Sleep/Wake button again to turn the device back on again.
4. Hard reset
Still having problems? Try a hard reset. Press and hold the Sleep/Wake button and the Home button at the same time until the Apple logo appears on-screen. This forces the iPhone to restart and will clear almost any glitch. If even this fails, however, you need the nuclear option…
The last resort is to restore your device from a backup (so tap Settings > iCloud > Backup and make sure iCloud Backup is enabled, and then tap Back Up Now if the iPhone has never been backed up).
If you need to restore, connect the device to your computer (using the cable will be more reliable than a Wi-Fi connection, especially if you're having problems), open iTunes and click the iPhone's name under Devices. In the Summary tab, click the Restore button, and confirm. iTunes will restore the device's software and settings and ask you if you want to restore from a backup – select the most recent.
6. Get expert help
If restoring your iPhone 6S or iPhone 6S Plus doesn't help or you can't do so, you'll have to get help. The best place to go is the Genius Bar at an Apple Store.
If you wish, you can book an appointment direct from your iPhone using a free app called Apple Store (not App Store) – search for it in the App Store and install it on your iPhone. Of course, if your iPhone is totally frozen, you won't be able to use this app, let alone search for it and install it if you haven't already got it.
Instead, visit from any web browser to book an appointment.