1. Use Airplane Mode "off"
If you need to save battery life, you can use the device’s Airplane Mode. Airplane Mode cuts off data, texts and calls but it will also conserve battery life in those dire moments when your iPhone 6 is on its last legs.
Click Open: Settings > Airplane Mode "off".
2. Turn Off Background App Refresh
You can turn Background App Refresh off for all apps or only for certain apps. If you see an app listed with background activity at the top, you may want to simply turn that app off.
Go to: Settings > General > Background App Refresh > Turn it all off apps.
3. Reset All Settings
The first thing to try is Reset All Settings. This does not delete your data or wipe the content from your iPhone 6, only the settings. If something is messed up this may fix horrible iOS 8 battery life in about 5 minutes.
Go to : Settings > General > Reset > Reset All Settings.
Note: If prompted, enter your PIN or pass code. Wait for the device to restart and then use your device like normal. You may need to reconnect to Wi-Fi, Bluetooth and change some other settings, but your apps, photos and accounts will all still be there, exactly how you left them.
4. Turn Off Push Email
When push is turned on, your iPhone 6 is constantly pulling data from servers, checking for new mailIf you don’t need email to instantly show up on your phone, Push is a setting that is probably worth turning off.
Go to Settings > Mail , Contacts, Calendars > Fetch New Data –> Choose the setting for each email account.
5. Turn Down Screen Brightness
You can toggle the brightness of your iPhone 6 display inside the Control Center, the menu that is accessible with an upward swipe from the bottom of the phone. It’s also accessible inside the device settings and you’ll want to head to Settings > Display & Brightness and toggle it there.
6. Turn Off 3G/4G LTE
Using the faster 3G/4G LTE network on your iPhone 6 can use more data and thus, cause battery life to plummet much faster than when connected to Wi-Fi. The drain will be especially noticeable if you are on the edge of coverage.Turning 3G/4G LTE off when you’re in spotty areas or when you don’t need it could help you save a ton of battery life
Go to: Settings > Cellular > Enable LTE -> Off
7. Restart the iPhone 6
You have not restarted the iPhone 6 at all since you picked it up or even if you’ve gone for a few days without restarting, go ahead and turn the iPhone 6 off then back on. Sometimes a simple restart is enough to stop whatever was causing bad iPhone 6 battery life and reset things so the battery will last all day.
To do this, hold the power button down until the screen changes, then slide to the right to power off. After a minute or two hold the power button to turn it back on.