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if you go back to the boss room, there should be a pink heart in the middle that you can walk over to unlock the dash

boss progress is saved after you defeat a boss, so you shouldn't have to complete the fight again, and item progress is saved when you pick it up, so if you did, you should be able to dash with "spacebar". If you can't dash, fast travel to the boss room (using the map "M" while in a pool), and the dash upgrade should be sitting there

Ok, nevermind, further testing has revealed a missing element. Thanks for the report! I'll fix it and publish the patch =D

They should be somewhere in your appdata folder. I strongly advise against editing the save files, problems may arise. There is a "reset progress" button in the main menu, however, so any fuckup should be resolvable by those means

Edit: nope, current version doesn't have reset button... I will publish a patch today