I also faced the problem forgot iTunes backup password for iPhone 6. But fortunately I find forgotten iTunes backup password out at last with a third-party iTunes backup password recovery tool, iSunshare iTunes Password Genius. It is recommended by my best friend. So I tried it, and unbelievably it really helped me and fast recovered iTunes backup password.
In the process, I find the as long as you find the password to open file Manifest.plist, you can restore your iPhone from iTunes backup successfully. Therefore, no matter your iTunes backup is saved on Mac or Windows computer, you can copy the Manifest.plist to Windows computer, and run iTunes Password Genius to recover iTunes backup forgotten password in few seconds.
Nothing needed to be worried about, especially password recovery efficiency and backup safety.
Just get the iTunes backup password recovery tool and learn how to use it. http://www.isunshare.com/itunes-password...
Note: Besides iTunes backup, actually you also could restore your iPhone 6S from iCloud backup. Or restore iPhone data with iOS Data Genius, which also provides two ways to restore data, iOS devices and iTunes backup. If you choose iTunes backup, you would also need iTunes backup password recovered with iTunes Password Genius.