Thank you Brett!!
Here I used the flat end of a spudger to unlatch the connector top.
In my iMac the cable was stuck to the structure of the computer with a piece of foam tape. Pulling it very gently, gradually it detached with no damages.
From this point onwards I found very useful a flashlight to illuminate inside the iMac.
Reassembly: be careful with the screws! The magnets attract them and the screwdriver as well! Better to use a screwdriver without replaceable end insert. I had to use one of this second type and I struggled a lot!
You need a big clean surface free, or at least 2 or more free surfaces because the iMac, the glass and the LCD display are quite bulky. You'll also probably need space for a tablet or a laptop to read this guide while you are working. And don't forget room to work on the optical unit and for the tools you are going to use. I covered a large table with a double layer of bubble wrap. In my opinion it's better to put the glass face down to limit powder sticking on the internal surface.