If users haven't backed up important files in advance, they have to suffer from data loss.
But luckily, users still have a chance to recover lost files no matter what upgrade method they choose, Over the Air or from Android Factory Images.
This update is rolled out over a period of time, and for most users is a seamless experience.
In most cases the OS will prompt the user with the message "The system was unable to unlock your login keychain", followed by these options…
Selecting "Continue Log-in" will provide access to desktop files, however network and some third-party applications will present dialog warnings because the keychain is locked.
For example: "I have a bunch of photos and other files that I forgot to backup before applying the update from Lollipop to Marshmallow, and now they are gone.
Do I still have a chance to recover them" In addition, using an Android factory image to upgrade Android device will wipe all data.