You need to copy-paste your downloaded save file into that folder with your current save (if you have one). If you already have a save, this will ask if you want to replace it. Replace it, and now you should see the downloaded save working with a fully unlocked gallery for battlefuck, defeat cgs, and endings. You'll also notice that the save file has significantly higher stats and is right at the final boss along with a pre-made build for getting Ending 1 on that final boss.
Just tested it like 10 minutes ago. On Win10, I just searched "AppData" with the start menu search and it appeared with the proper "Local" "LocalLow" "Roaming" folders, so that might be a little more convenient or not for you to reach LocalLow.