1. Plug in your power cord to the device. Some updates require you to plug in. (Not necessary if you have sufficient battery life)
2. Make sure your device is connected to WIFI.
3. Tap on Settings
4. Tap on General
5. Tap on Software Update
6. Tap on Download and Install button for iOS 7
You will need to tap Agree to Apple's Terms and Conditions . Give time for the update to download, install, and reboot. Time may take between 4-10 minutes depending on device age, size of drive, number of apps, and WIFI speed. Once the installation is complete you will be prompted to configure your device.
After the update, you will probably need to login to App Store, iTunes, and all other accounts on your device(s).
Remember: a new iOS will affect apps that are not updated to run on iOS7.