Why isn't my phone updating?

[AC Question]

I have lenovo a6000 plus phone I downloaded the system update but after that it doesn't install and shows android system recovery (3e) massage . It give option reboot system now, apply update from adb,power down etc I trying every option but it not work now what can I do . I does not try wipe data/ factory reset because in phone memory there is some important photos and video's