That center nut needs to stay ON until the strut is out of the car and in a spring compressor to hold the spring our you can have a dangerous event of the spring going thru the air with some force. You take off the 3 nuts holding the top of the strut to the body and take the full strut assembly out.
Here's some video info:
https://www.youtube.com/watch?v=WFuERvwEt-I
That top nut is usually a Nylok or self locking type so it just needs to be tightened up properly, It probably does have a specific torque value.
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