How do I change the look of an object in my game?

I made a game completely following tutorials using a sphere as my main character. Now I have downloaded some free assets and would like to change the look of my sphere to a person. After messing around with mesh colliders, filters, sprite mask, etc. I tried to find a solution on google but haven't seen anything. It's odd since I've seen many videos of people starting off their game with basic prototypes so there must be a way to change it but I haven't found it.