I am curious if we will have to install iOS 9 on our iPhones? Or can we opt out?

I am not asking about a problem per se. I am just curious if we will have the option to "opt out" of installing iOS 9 on our iPhones. I have an iPhone 5S 32GB, with the latest 8.4 (?) updated software. And I still have about 22GB of storage left on it. I also have used almost none of the allowed 5GB of iCloud storage.

My desktop computer uses the Windows operating system. (For now anyway. I am contemplating buying a new computer, and since I have an iPhone, an Apple computer of some sort is high on my list. Plus several of my friends have told me I would be crazy to buy anything else.😕) I am asking about iOS 9, because I recently upgraded to Windows 10 on my PC, and really wish I hadn't. It is so completely different from the Windows 7 system I was used to, and I am not the least impressed or happy with it.

I was reading about the changes with iOS9, and am a bit gun shy about another system upgrade. So, that is why I am inquiring about if we will be allowed to opt out of the upgrade. And if we do, might it affect how well the iPhone operates?

Thank you to anyone who might answer this. 🙂

Barb M.