Bliss Pop Custom ROM update !

Hi Guys I wanted to ask that I have LG G3 D855 and I am running a custom ROM Blisspop v3.7 . I am getting update v3.9-d855 12th August and Its asks me whether to download or ignore.. Now the thing is if I download this file on my G3 so would it update the ROM automatically or I have to go to TWRP recovery and flash the update and the Gapps... ?? I am attaching a screenshot to show you.
Please let me know as I haven't gone through this thing before.

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