Google announced a bunch of new features in the upcoming Android N during the Google I/O keynote, one of which was seamless updates. With this, new Android devices running N will be able to download the updates in the background and the next time you restart the device you will be running the new version.
Turns out, the key word here is ‘new’. According to Android Police, the seamless update features won’t be coming to existing Android devices.
The feature requires two partitions, one offline and one online, and it installs the update on the offline partition and then switches it to online when the device reboots. So essentially you need two partitions for this to to work, something current devices don’t have. Creating those seems to be too much work, something Google does not want users to go through (and risk damaging their device).
As such, the only way to get this feature now will be on upcoming devices running N that will be launched with this feature enabled.