Can't update iPhone4s to ios8?

My iPhone is currently on ios7 but how do I update it?

Settings > General > Software Update
or
Plug into computer, run iTunes

You can't update the 4S to iOS 8. Only iPhone 5 or higher, sorry mate. I'm sure if your phone is Jailbreaked you could update it illegally (lol)

The best thing would be to update through iTunes on your computer. Make a backup of your data before you continue.

You can, alternatively, do an Over The Air Update as Scott says.