How update my iPhone to iO5 have looked at iTunes but no luck

I have found other posts but they have not answered my query. How do I update from version 4.3.5? I wld like iO5 version, and then update to iO6, but I am not overly tech savy, & suggestions to reset etc do not make much sense to me without further instructions! I am a gr8 'map reader' ;) I an follow directions :)

Any assistance wld b fab :)