audioOS 12.1.3 (build 16D39) is an official Apple over-the-air update for the HomePod, released on 2019-01-22T18:10:06Z. The update package is 1.4 GB. This is a delta (partial) update that requires 11.3 already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: e7591074ccd9f489c73f6ba0bfdcb7936eab804f.zip.