I have my iphone 4S with IOS 6.1 how can i update my iphone to IOS 7?

As i have seen videos poeple saying that update from 6.1.4 if so tell me how because in my itunes the update is showing 6.1.3

IOS 7 is not out yet however you are one behind still as the current one is 6.1.4

IOS 7 will be out by july-august

You shouldn't update to iOS 7 now, it's not stable but if you like you need add UDID to developer account, there are some tutorial guide update to iOS 7 without UDID but it's not safe, if you do a wrong step, that's a big problem
You can request someone have a developer account add UDID for active your device, you can search in Fiverr or fivesquids.co.uk
Something like this:
http://fiverr.com/apple_review/activate-your-udid-to-have-ios-7-on-your-iphone-within-5-minutes

Sorry iOS 7 is not available till autumn but here is a way of getting it