and how can i do this effect

That was pretty seamless. I am guessing the original clip was recorded and is what is played on their timeline up until the reflections appear. The first frame with reflections could have that original clip being played back on the TV and the camera pulls away from it to a different live shot in the same room. Special care would be needed for the starting camera position during the second clip but a two frame cross fade is all you need to cut between them.

If this was a composite done in post they did a great job with motion tracking and adding the reflections on top. I would love to know how they actually did it.

Cole
 
It's just a simple morph transition with the video in the TV and reflections added in post. It doesn't look anything like a filmed TV screen after the transition and if you watch the girl in the bottom right you can see the morph/change in perspective on her face.
 
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