This is the rated 18+ patch for Bifrost.
Link to patch
You can find your game folder’s location by:
Go to your Steam Library, right click on ‘Monster Girl Club Bifrost’, click on ‘Properties…’, go to the ‘Local Files’ tab and then click on ‘Browse Local Files…’
The default location is usually
C:\Program Files (x86)\Steam\steamapps\common\Club Bifrost
Just drag the file in the zip to the location above and enjoy
To verify it worked, you will see a lewd title screen when launching the game again