To update a device from to Android 7.0 (Nougat) , you should first check for an official update in your system settings, though most older devices will require a "Custom ROM" since official support for version 4.4 has largely ended. Option 1: Official Update (Easiest)
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If your manufacturer stopped supporting your device at version 4.4.2, the only way to reach 7.0 is by using a like LineageOS (the successor to CyanogenMod). Community developers often "port" newer Android versions to older hardware. To update a device from to Android 7