To get to it you first have to remove the cover around the gear lever. This is clipped in and does not want to come out immediately. But with a little perseverance and caution, it will come out in one piece.
After this is done, you can carefully push up the housing around the gearshift lever. The emergency release will then be visible on the left side. You have to press this, then you can change the gear from P to neutral at the same time. Now the smart is rollable again.
To get back to P, you have to press the yellow lever again.