You can only download and install the highest iOS version that a device supports. If your iPhone is a 4S as your tagline suggests (in which case it must already have iOS 5+ on it as that is the lowest iOS version that it shipped with) then you should have a Settings > General > Software Update option on it for direct update - a 4S then you can only update it to iOS 7.0.4
If your iPhone isn't a 4S and isn't already on iOS 5+ then you can only update via a computer's iTunes - but what iOS version that you can update to will depend upon the model.
To update via a computer, connect the phone to your computer's iTunes and copy any purchases off the iPad to your computer via File > Devices > Transfer Purchases. You may also want to copy photos and any important documents off the iPad as well e.g. via the file sharing section at the bottom of the device's apps tab when connected to iTunes, via wifi, email, dropbox etc - they should be included in the backup, but it's best to have a copy of them outside of the backup just in case. You can then force a backup of the phone via File > Devices > Back Up.
Then start the update by selecting the phone on the left-hand sidebar of your computer's iTunes, and on the Summary tab on the right-hand side clicking the Check For Updates button
Updating to iOS 5+ : http://support.apple.com/kb/HT4972