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Open up editor.exe.
Go to Set Metadata.
Go to [Global] (It's the the top one).
Change PlayerA and PlayerB...
Where it says Sprite="Red" or "Crystal", change that to the filename of your custom sprite. You're free to change the bicycling sprite, surfing sprite, etc. this way as well. Just change the other fields to what sprite name they need to be.
I suggest you take a read at the Notes for anything you might have further questioning on, or looking inside the comments inside the game (Events) to see where certain things point to. https://upokecenter.com/projects/pokestarter/notes.html
Seeing as though you've already gotten help, I'm going to lock this.
If you have any further questions regarding Pokémon Essentials, please post in the thread Starrodkirby86 mentioned.