RPA file is a type of archive.
If we are talking about the game, then you do not need to install anything, just unpack the zip archive and run HerNewMemory.exe. https://zodiacus-games.itch.io/hnm
If we are talking about BDSM DLC, then you just need to extract the zip archive into the game folder. It should look like this: Her New Memory/game/bdsmdlc.rpa