It can be found as DLC for the game on the Steam Store: Duel Girl - 18+ Patch
If the Steam Store page isn't working, you may be able to install it by either clicking the following command or running it directly on your PC: steam://run/1120180
Note that this will immediately run the game and start downloading the DLC if possible - you may need to close the game and wait until the DLC finishes downloading for the patch to work.
Steam store: https://store.steampowered.com/app/1116870
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