1. Download the patch from Denpasoft: here[denpasoft.com]
2. Make sure you have ran the game at least once from Steam
3. Extract the files to a folder of your choosing
4. Run the .exe file. The readme may be blank. Just click through.
5. You're done!

If you have installed and run the game already but the patch still says "9-nine-:Episode 2 were not installed", you can extract the installer (.exe) file using 7zip and put the patch files into your game folder manually

1. Right click at the .exe
2. Move your mouse over "7zip"
3. Click "Open Archive"
4. Move "adult.xps" file into your game folder
5. Done!

In case you didn't see "7zip" option when right-clicked, it is very likely that you haven't installed 7-zip yet. You may download the installer that matches your operating system and install it from https://www.7-zip.org/download.html