Here's how you can quickly turn off Wi-Fi on your iPhone to prolong your iPhone's battery power.
1. Swipe up from the bottom of the screen to open Control Center.
2. Tap the Wi-Fi icon to turn it off if it's on.
Or you can do it a different way:
1. Go to Settings.
2. Go to Wi-Fi.
3. Toggle the Wi-Fi switch to the off position.
Wi-Fi is extremely useful but only when you actually need an internet connection. Otherwise it just drains your battery down for no good reason.
Additional resources for iOS