How to update iphone 3gs to ios 6?

I have an old iphone 3gs which still works, but i've never updated it from when i first got it, so i have no software update button on my settings in general. And everytime i try to update it through itunes it says it hasn't got enough memory when it does, please help

You need to delete some software or anything you don't need from your C drive it needs nearly 1GB free space to update to ios 6. Then connect it to itunes and then you will see an update button click on that and it will do the rest. It will need internet conncetion as well.