IIRC, older (retail) versions of Sins had a launcher of sorts. Once connected via a Stardock account, it should automatically notify you of any game updates. As usual, Steam versions of the game should automatically download the latest patch.
More info here: https://www.sinsofasolarempire.com/update
Note that if you bought the copy used, it is possible that its key was already activated by another account, in which case, as far as I know, you're screwed.