Why cant i change the ios?

In the beginning I just want to say that my english isn't very good but I hope that you will understand everything and help me with my problem.

I use iPhone 4s ios 5.1.1 and I want to chagne it for ios 6.1.3 or 7.0.3 but I can't. I open iTunes, click on update with buttom ctrl (windows) and I can't choose the ios 6.1.3 or 7.0.3. There is only 7.0.4 but I don't want it. So please tell me what can I do. I hope you will answear asap. Thanks!!!