I keep looking at this thread hoping somebody with more knowledge than I will answer it. As I understand it, the service spanner on the dashboard information panel can only be reset with the manufacturers plug in diagnostic tool. I don't know, but I think when you have gone about a thousand miles over the scheduled service it will reset itself.
This is what it says in the handbook, which can be obtained from the download section of this forum
SCHEDULED
SERVICING
(where provided)
This warning light turns on to indicate
servicing and it will stay on glowing
steadily until reaching the deadline. The
warning light will turn off after carrying out
servicing at Fiat Dealership or after 1000
km at servicing deadline.