If Windows asks if you want to replace existing files, select Yes . The patch overwrites the "clean" game scripts with the adult versions.
If you prefer not to buy the DLC through Steam, you can obtain an official patch from the publisher, . Nekopara Vol. 1 Steam 18 Patch
For the definitive experience
. This DLC restores the adult content that was removed from the standard Steam version to comply with older store policies. Content Restored If Windows asks if you want to replace
The 18+ patch restores content that was removed to meet Steam’s general distribution guidelines: : Full nudity in bath and sleeping scenes. Nekopara Vol. 1 Steam 18 Patch