How do I revert to prior iphone OS from the phone?

I have an iPhone 6 and after reading all of the requirements, and reviews, I just made the HUGE mistake of updating the OS to 9.0.2 and I cannot figure out how to revert to the prior OS version from the phone itself.

I have an older Macbook, (13" Mid 2010 model) running OX 10.6.8 and iTunes 11.4 (never updated through the rest of the "big cat" series, or Mavericks or Yosemite because of all the bad reviews regarding stability and speed on older laptops). Problem: I now can't synch my phone, because 9.0.2 requires a newer version of iTunes, which requires a newer version of the OS. (No notice about that when asked if I wanted to upgrade, thanks Apple.)

I only use the laptop for basic word processing, spreadsheets, email, and photos. I'm not a gamer, I don't use social media. I don't need all the bells and whistles of a new OS that just bogs down the basic processing. Same for the phone: used for music, text, email, and as a phone. I don't need all the additional crap packaged with this OS.

How do I revert to the prior OS version from the phone so I can get back to work without spending an entire day upgrading everything the grand scheme of Apple requires to restore basic functionality? All the online help I can find requires ITunes to recognize the phone to restore a prior version, which I can't do since ITunes won't recognize the phone. Thanks in advance for any help you can offer.