The iPhone can connect to the Internet using either a cellular data network or Wi-Fi. When Wi-Fi is activated, the iPhone will use power to seek Wi-Fi hotspots. When you know you're not going to be near a Wi-Fi hotspot, turn off Wi-Fi so the iPhone won't seek hotspots--conserving battery power.
Tap “Settings” on the home screen of your iPhone.
Tap the “ON/OFF” switch to the right of Wi-Fi until "OFF" appears with a gray background to turn the Wi-Fi off. The iPhone will stop using Wi-Fi networks to connect to the Internet, and the Wi-Fi status bars in the upper-left corner of the screen will disappear.