If you're running iOS 7 and want to update to the latest version, there are a few different ways to do it. The easiest way is to go into your Settings app, tap on General, then Software Update. Your device will check for updates and prompt you to download and install any that are available.
You can also connect your device to iTunes on your computer and update that way. Just make sure you have the latest version of iTunes installed first. Once it's connected, select your device from the sidebar in iTunes, then click on the Summary tab. Underneath the Check for Updates button, you should see an option to Download and Install - click that and follow the prompts.
If neither of those methods work for you, you can always try downloading the IPSW file for iOS 7 manually and installing it with iTunes. You can find links to those files on Apple's support site or various other places online - just make sure you get them from a reputable source. Once you have the file downloaded, open up iTunes again and hold down Option (or Shift on Windows) while clicking on the Check for Updates button. A window will pop up asking you to select an IPSW file - choose the one you just downloaded and follow the prompts from there.