Disable data encryption on an Android device by using the Security section of the Settings application. Some devices may not have the option under Security and thus require a complete factory reset. Make sure the battery is at least 80 percent full and the phone is connected to a power outlet.
Open the Settings application, and then tap Security. Depending on the exact device, you may see a Decrypt button in this screen. If so, tap the button to have the phone reboot and change the settings. This process takes approximately 15 minutes.
If your phone does not have a Decrypt button, open the Settings application and tap Backup & Reset to reset it to factory settings, then tap the Factory Data Reset option at the bottom of the menu. The phone then performs a full reset and may reboot a few times during the process. As a factory reset method also removes all personal data on the phone, make sure to create a backup before initiating the reset. If using an Android 5.0 or newer, it's likely your settings are tied to your login credentials and should update automatically.
The Nexus 6 and the Nexus 9 by Google do not allow the user to disable encryption in any way. As such, factory reset on these devices only results in data loss.