How To Swap Assets? (Generic 2D Shape -> Custom Image)

I have a game template I'm attempting to edit, here's how my character looks:



I'm trying to gain some experience swapping out assets and making edits. All I want to do is swap his head out for a custom image. I did some searching but didn't find what I need. I saw some plugins but I think it might be good to learn how to do it manually first?

I thought maybe I'd drag an image into the sprite or material section under the inspector tab but it's not letting me.






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Edit: I seem to have figured it out!