Board Thread:Questions and Answers/@comment-30088034-20170108151825/@comment-27701762-20170109235750

It will be the first of those two folders. Open that folder, and in there should be various files. One should be called "undertale.ini," and then there should be several files with the format "file#," usually "file0," "file8," and "file9." Delete those files, and you'll be good. They should not be hidden files, but if you open the folder and see nothing then try going to the "View" tab and checking the box that says "Hidden items." That should reveal the files.