Lets face it. In a race between a moron and an idiot I would come 1st.

CobraX V1 Goggles were working fine, but I saw new snazzy features, so I did 3.4.6 firmware update.

Yes there is readme. But its garbled characters.
Yes I saw the youtube video.

Afterwards menu was messed up. Flashed 2.4.0, menu got fixed. But no DVR playback or recording.
So, I flashed 1.3.9. DVR still not working!!

How do you fix this?

I know it's not a very helpful answer but contacting Skyzone support regarding your issue might be your only option. Then wait and hope that someone actually bothers to get back to you.

https://www.skyzonehobbies.com/support.html

Don't fix what's not broken!!!

If an update becomes available, with features you want, put a post up on News about, leave it a couple of weeks to see if it bricks anyones goggles and then install it. (I'm a bad person, but seriously don't install anything until it's been in the wild for a few weeks).