How can I take off the sensor from the back plate when opened?

I had the same Problem.

You will need to break the four black points, take it off (there is a square ring of adhesive for the middle sensor, that you will scratch off). The new backplate had a new adhesive ring, and there are four black pins, where the new sensor is mounted on. And with heat (I used a solder iron), you can melt the pins and they will then hold the sensor in place, the same way the old plate did.

See the battery replacement guide - Huawei Watch 2 Battery Assembly Replacement

Step 7. Looks like the ribbon cable pops off at the connector.

I have the same problem. The battery tutorial doesn't help. The question is not how to loosen the ribbon cable, that's easy.

The question is how to remove the sensor and other parts from the old back cover (right on the photo) to the new back cover (left on the photo).

Anyone know how to do this?

The soldering iron was excellent advice! I just performed this delicate surgery myself and used an Exacto knife to slice the top of the old plastic posts off that hold the sensor disk in place. Then it was just a matter of prying the sensor disk off (although that middle square was stuck on there pretty good). I smashed the sensor disk down on the new backplate after aligning the posts and melted down the posts slightly with a soldering iron and the sensor disk is staying in place great. The magnet or vibrator motor or whatever that is at the bottom took quite a bit of effort to remove and transfer (I'm guessing it's the vibration motor as there are two tiny electrical connection arms at the sides, and after I removed it the old backplate would still stick to the charger). Eventually I cut enough of the old glue and pried back enough of the surrounding plastic rectangle that I was able to finally pry it up. I then used a little hot glue to secure it to the new backplate. I am extremely happy to report that the watch charges on the first try now, every time. No more fighting, wiggling, clamping, or reseating required which is the battle I had been fighting for quite a few months.