You won’t be able to assign skills or respec them until you’ve advanced the game to the point where you reach the Moon Snail’s Domain. You’ll see an option to “Refund Skills” when speaking to the Moon Snail. However, this will cost you a Shark Egg, which is an exceedingly rare item that you can’t get until you’ve progressed a little bit further into the game. The easiest way to get a Shark Egg is to head over to the Prawn Shop in the lower secti
