Click the track's name on pretty much any page. If you uploaded the track, there should be an "Edit" button
Editing of basic info is displayed, with a link to Edit Files
Important: Tracks will not appear in playslists without a file nicknamed "preview". This file can be a .FLAC, but it's generally more performant to use mp3 for this
Change any nicknames you need to, then "Save Nicknames".
Click the outlined area to select file(s), or drag-and-drop into this area from your desktop
After the success message, new files are in place and ready for nicknaming.