Then you can't. You need to do it when it's still bootlooping. Connect to a computer and run adb logcat. In addition, you need to have USB debugging enabled before the update.Sorry how do I take logs during bootloop? I have the phone up and running already.
I've been scratching my head and trying to trigger the crash. I've formatted data and clean flashed the previous build, finished the setup, let everything settle down, and then update to the new build through OTA. I've done this 3 times this afternoon and have not been able to trigger a single problem.