How to do a factory reset on my iPhone without having to update iOS?

I own a 4S with iOS 7.1.2 on it and IIm about to restore it but I wouldn t like to move on to iOS 9.2 or whatever that s the latest one. I've heard some ways around it that you can like create a backup, restore and then use that backup to get the OS back, however IIm kind of sceptic about that method. If that was the only way I d be down though but since you guys must know something I thought I d first ask some experts. So how do you think I should work through this without losing my current operating system?

Okay, here's a little update of what's going on, I've done it and it didn't update so yea, rumors were wrong or something, I'm still running iOS 7.1.2.