Create Your Sprite
The first step to changing your default sprite, is to create a new sprite. You can import an image through the Scratch Paint Editor or draw one by hand. You may also add scripts,costumes, and sounds. These will all be saved with your sprite.
Saving The Sprite
Once you have your new default sprite ready, right-click the sprite in the Sprites Pane and click "export this sprite". You will now be in the Scratch File Manager. From here, save your sprite to the costumes folder with the name "default". Restart Scratch and your new default sprite will have replaced the original Scratch Cat.