Ahoj, Lubomir. Your video is actually way better than some of what I have seen in the recent past. More than good enough to provide comment what is going on. Your squats are all a little high and you are correct that you are rounding your upper back. With some work, you will probably be able to correct all of that. Firstly, you probably want to take 5 to 10 kg off the bar and get depth on every rep. You are rounding primarily as you begin the ascent. This can be a tricky problem, so I am going to give you a couple of cues to try. Firstly, don't overdo the looking down thing. Keep your eyes on where the rack meets the floor in front of you. Right now, you are looking down at a point maybe one third of a meter ahead of your feet. Cue yourself to keep your elbows down on the ascent. Squeeze your shoulder blades together right before you get to the bottom and hold on to that as you ascend. Think of yourself as maintaining good posture, even as you lean over. Also, check this out from our friend, Bill Hannon:
Identifying and Correcting Thoracic Spinal Flexion in the Squat | Bill Hannon