I'm not sure if by upgrade you mean a boost to performance, or just any modification.
The first mod I did was Rally Armor mud flaps for extra protection, and to avoid a lot of nasty spray on the sides of my car as it rains a lot here; I added gloss black caps with washers to cover the screws
Almost in conjunction with that I got the car opti-coated; I struggled with putting a good chunk of money towards that for "protection" versus spending that money on the fun stuff, but I have no regrets about that choice now.
For performance - I did the Apexi drop in filter about two months after owning the car