If you're wondering why Steam is downloading an update to Sepia Tears for the first time in forever, it's because we're finally getting ready to launch the Reprise Edition DLC! The Steam page isn't public yet, but in the meantime, we have some new achievements for the base game, as well as new icons for the old ones. The game is the same as before, but there's one new achievement per chapter.
If you still have your old save file, the game should automatically unlock the new achievements when you boot it up. The one exception is Unannounced Visitor, because that one is part of the Day 4 branch, so it's technically missable.
If this doesn't work, you might have to delete your old save file and just ctrl through the game. I've added a Delete Save Data button to the options menu to make this easier. If that's buggy, you can delete the save directories manually. Remember to delete BOTH of them.
The scripts for the original game are somewhat disorganized, so please let us know if there are any bugs in this update!
EDIT: It looks like the new update might be incompatible with the old saves. People have reported an error message on launch, which is the same as this Ren'Py error message for National Park Girls. Unfortunately I don't know how to fix this because Ren'Py handles saving/loading on its own, so for now you may have to manually delete your old files. Apologies for the inconvenience.