Which version of Sins did you have? How you update the game depends on when and where you bought it. Early on you registered it on the then Stardock owned Impulse program, which is now done through the Gamestop PC downloads app. If you registered in then it will still be their when you sign in to your account.

If you bought the game on steam, it's obviously still on your steam account. And if you have Sins trinity, you can download the entire updated game directly from the Stardock website.

Then you were here even earlier than me, but you'll need to register you game if you want the latest updates. The last update that was released without needing a download client was patch 1.05. Original Sins is now on version 1.191, or at least 14 patches later.

If you know you registered your game at the time, I would guess you would update through the gamestop app, but you may want to email support@stardock.com to make sure if you'd rather not get something you don't absolutely need.