I am new to Unreal, coming from Unity.
I’m using the PaperCharacter character/pawn for 2D movement, following this tutorial (How to make a 2D game in Unreal Engine 5 - Beginner Tutorial - YouTube) but the default capsule collider is not the best shape for my character sprite. I need a square-shaped collider for the square-shaped character I’m making.
How do I change out the default capsule collider for a box collider?
The best thing you could probably do is by just adjusting the size of the capsule collider to the sizing of the character. you can do this by selecting the ‘‘capsule component’’ and within the details penal underneath ‘‘shape’’ to change the ‘‘capsule half height’’ & ‘‘capsule radius’’ adjusting these values will make the capsule component to be more square/rounded shaped 
if you REALLY want to have a box. then just add a ‘‘Box collision component’’ and adjust the collision presents to your needs