Download firmware 40.134 which is Android M. Once the Zip file is downloaded, place the file on in your internal storage, and restart device. The update pop up should show. Its recommended you backup files and factory reset before you update. And factory reset once after it is done.
My Phone ( ZE550KL stuck ..Bootloop ) I did ' apply update from SD and Sideload via ADB ' but this doesn't solve my problem, stuck in ASUS Logo and it keeps on ReBooting.
My device is Asus Zenfone 2 551ml. It's battery is draining so fast that in 2-3 hrs it goes from full 100% to zero. Even now it's not charging fast. It's fast charging is not working . I don't know what's happening.