If you already own the original BioShock 2 on Steam or GOG, the Remastered version is typically added to your library for free automatically.
Remastered versions of the series are frequently given away for free to Amazon Prime members. BioShock 2 Remastered Free Download (v31.08.2022)
The BioShock: The Collection (including the remaster) is a recurring free title on Epic Games during their seasonal giveaways. If you already own the original BioShock 2