Firstly just go into each of the levels by clicking on them in the menu, head back to the menu, and repeat for all levels. This will unlock 99%/100% of all achievements.

If 1 or more achievements are stuck, we'll be downloading an old version of the game.

This follows this guide, and is OK'ed by sites like AStats.

First, open the Steam Console by pasting this into your browser's address bar (doesn't work on Chrome):

This will open up next to your username.

Then, paste this into the console:

Once it has completed ("Depot download complete"), go to the directory it shows:

Copy the single .exe in the folder, go to the directory where you have 'sudoku.' installed, and replace the .exe there with the one you've just copied.

Now, launch 'sudoku.' from Steam as usual, and click on the levels via the menu that previously didn't work.