How to download the latest iOS to my iphone?

I'm trying to purchase a ringtone on iTunes and it says I must update the latest iOS an I don't know how or what that is.
I have an iPhone 4

1)download and isntall the latest version of itunes on your computer
2)plug in your iphone to your computer and open itunes
3)make sure that your are connected to the internet
4)if your iOS is not the latest version itunes will notify you
5)from the notification click on download and isntall
6)wait until itunes finishes downloading and updating your device software

P.S: if you are on ios 5.0 and later and your iphone is not jailbroken ( does not have cydia installed)
you can update your software from settings-->general-->about-->software.(t… Is less time consuming it will take about 10 minutes)
if you will be updating your iOS from itunes take into consideration that this process will take up to 2 hours depending on your internet download speed