Is it possible to edit the description of a pin? As an example, I would like to mark all files that I later want to export with, say, a blue pin and would like the description in the collections list to say "Blue - export".
Nope, sorry.
That's what labels were designed for. Many workflows use labels like "For Review", "For Export" or "For Printing" for exactly that purpose.
Yes, I can use labels, thanks. Don't know why I didn't think of that ....