By default, your iPhone 6 locks automatically after one minute; its touchscreen locks and its display turns off. To prevent this, change the auto-lock time. If you choose the "Never" option, your iPhone never locks automatically. Be aware that you may accidentally dial numbers, send texts or change settings if the phone's touchscreen doesn't lock automatically.
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Changing the Auto-Lock Time
Open the Settings app on your iPhone from the home screen or the Apps screen. Tap "General" and then tap the "Auto-Lock" option to view the automatic lock settings. Change the auto-lock time by tapping one of the options -- you can choose between one and five minutes. If you choose "Never," the phone will never automatically lock; its touchscreen will remain active and its display won't turn off. Note that this setting has a negative impact on battery life. The new Auto-Lock setting is applied immediately; you don't need to restart the phone. If the phone doesn't lock automatically, restart it. If the problem persists, you may need to reset your iPhone.