That looks like #10 in the photo?
View attachment 140919
I would start with page 65 and read through the GranCabrio manual to see if you really want to tackle it. There's a convertible top release latch in the trunk that will allow for manual operation of the top.
I believe I read you should wait 10 minutes after shutting off the car (roll windows down first) for the pressure to be released and allow you to pull the latch to manually adjust the top.
It looks like beyond just the cable, probably some light interior work to make all the corners and such adjust back correctly too...
If it was me, I would find the best place for the part and find a convertible top/vinyl shop to fully fix the top. They may suggest removing the top entirely before working on it, so it can all be put back in the right way. When you're working on things like convertible tops, some parts can be like working/adjusting body work. Other folks that do this stuff daily are going to be 100 times better at getting things perfect.
If you're simply trying to swap the cable out, you may be able to do it with the top connected, and not bother with cleaning up the carpet/sides. I'd probably go the route of taking it to a professional shop that works on convertible tops.