Problem with reinstalling and updating game

I bought the galciv2 ultimate edition (in physical CD form) a few years back. The map editor was crashing on me a lot recently, so I uninstalled it using the uninstall program feature on my computer's control panel section.. After I reinstalled it, I wasn't able to use the update button in the program menu. I was told by the gamestop app that opened up that I needed to register the game. However, I couldn't find a way to do this.

I still have the manual with it's product key for the game.

Please advise.

Thanks in advance!