Once the new 100% file is in place, boot up the game. You'll be able to test the Super Characters on Dante Must Die mode right away. If you run into crashes, verify the file in the correct folder and double-check that Cloud Saves are disabled.
Before replacing files, find your original save folder. It is typically located here: C:\Program Files (x86)\Steam\userdata\[Your Steam ID]\601150\remote\ devil may cry 5 100 save file the game
Download your chosen 100% save file from a trusted community source (such as Nexus Mods). Extract the downloaded ZIP or RAR archive. Once the new 100% file is in place, boot up the game