How can I manually update the R8000P firmware

Have been very frustrated not only with the performance of this router but by the lack of clear support documentation here at Netgear.  Is getting a simple, three or four step listing of the instructions to update my router too much to ask?  (Not complaining at the community, just frustrated that a simple thing like this couldn't be explained without my coming here to beg for help.)

I have a .zip file for the router, but want to check that I unzip it and use the .chk file within as the firmware file.  Often firmware's a .img file, so I'm loathe to try to install that file and possibly brick my router.  Note, I am doing this on a wired desktop connection running Linux (Lubuntu) -- I don't want to install another useless app on my phone.  There's a section in the settings for updating the firmware, but the automatic one fails -- it just hangs at the "Getting GUI, wait one minute" and that's all I get before the thing times out about 10 mins later.

I assume I just go to the settings, navigate to the .chk file, and click "install" or some such, and wait until it installs.  The manual I have doesn't seem to have any section for updating the firmware, and the Readme simply points me to the manual.  

Thank you in advance for any clarity on this.