Hello, you can find the patch here too https://denpasoft.com/product/highway-blossoms-18-dlc/ named adult.rpa
Just copy/paste it in the folder where you installed the game itch\apps\highway-blossoms\Highway_Blossoms_Remastered-2.54-all-ages-win\game (it should have some other files with .rpa extension in it)
You should now have the option to activate R18+content in the preferences settings in the game, if the file is at the correct path.