simple question i guess my lights came on my dash to change my battery,

but what size battery is it, and is it just the one or two, and is it just alright to open it and swap?

Buy from dealer who will fit for you(act dumb....not literally just about thying to open the fob case)no need to reprogramme.

it is iver a 2032 or 2016

this means 20mm wide 3.2mm thick
or 20mm wide 1.6 mm thick

being fiat likely to be 2032 available at local hardware shop or super store
no reprograming needed as the battery is just for the central locking
the key code chip is powered by an arial around the ignition barel
it is easy to change with a small screwdriver
make sure you dont get finger grease on the contacts of the new battery as this will cause it to run down quicker use some tisue paper to manouver the batery into place