I was debating adding a button for that this last version, but if you're on windows then your save directory is in %appdata%/SternlyWordedAdventures
Your metaprogression, stat-totals, and other permanent flags like for one-off tutorials are stored in persistentSaveData, if you delete that you should also delete mainSaveData and combatSveData; those are your current character (or you could get them eulogised first), since they could put your metaprogression in a impossible state. If you played pre V40, deleting persistentSaveData will have it try to import the save data at %appdata%/WordTileGame
Deleting userConfig would, as you might imagine, delete your settings.
You could preserve the contents of saveStats and get the majority of the new player experience; you get a predefined single hero option with an empty stats folder (the adventurer with no curses or boons), the halfling unlocks on a non-empty stats folder, and world seed is based on the number of stats entries you have, so if you keep your stats history you'd instead get 3 hero choices with random curses/boons. You'd have a very high chance of rolling mechanically the same as the starter with no unlocks/seen items though, and you could technically get that experience as a full newbie by just going to the arcade first.
Whether or not you delete any stats, if you delete persistentSaveData you should also delete statsIndex; it'll trigger a recount of global stats and reindex them.