Question about screen replacement.

So I'm going to change the screen on my 6 plus, but when I got the phone, the screen was very badly damaged and I can't see anything on the screen. I can't power down the device because the screen is so bad, and doing a hard reset doesn't seem to be doing anything. When I press the power button a couple of time, the screen will light up with a few lines on it, then go dark. I've been trying to get it to die now for a week, but I can't tell if it's totally dead. Can I just disconnect the battery and do the screen replacement, or do I need to be positive that the phone is dead and powered down? Any help would be much appreciated. I don't want to cause any extra damage. Thanks.