I have iPhone 4s I want to upgrade my phone to ios7 how to do this if possible
How to download ios7 for iPhone 4s? - 1
It's not possible. Apple made it to where if you want the latest iOS, you have to buy the newest device. Sucks, I know. So does Apple.
What the other person said about Apple forcing you to use the latest hardware is BS. The iPhone 4 came out over 3 years ago, and Apple will be fully supporting iOS 7 even on that device. Meanwhile, updates for Android phones get delayed or never come out because the folks that make the phones and the wireless carriers keep messing it up.
iOS 7 won't come out to the public until the fall, but you can install it early if you register your device's UDID with a developer account through a site like http://www.udid.in/.It's $5, and then you can try out the new OS before everyone else. That's how I got it on my 4S.