having trouble updating to 7.0

hello:

So I went back to my nexus 6, changed the battery because it wasn't holding any charge; and in the process of that, the down volume key is very sensitive, and the power button from time to time doesn't work. I'll be getting those fix in the upcoming week. I got the OTA for nougat, download it and pressed the restart and install button; but when it restart it goes straight to the bootloader (the one with the android icon with the belly open), so I press start and when it loads up, it is still on Marshmallow. Any idea how to install it while the buttons are doing this??

Thanks in advance