Default Player Pawn - how to change material?

When you launch a game, you can see in the mirror a sphere.
That sphere seems to be the default player pawn.

I tried to find some step by step tutorials - but so far no luck. I was not able to find a comprehensive guide that explains step by step how to exchange the default pawn with a new pawn. I was able to create a new pawn and also change the world settings to make sure my new Game Mode will be used for the world.

But after running the game, I can’t move. And I don’t know how to assign the standard controls to the new generated pawn.

Second Idea:
If I can change the material of the default pawn, I could map any texture I want to (smiley face would be hilarious… or maybe frankenstein face when you look into the mirror) haha…
what ever… or I could make sure the material is 100% translucent / opacity and transparent.

If I can make the proper material change, I should also be able to make the sphere invisible.

The question is:
How can I find the material of the default player spawn?

Or… like I mentioned before… how can I swap out the default player spawn with my own and make the new pawn invisible for the game?

I found bits and bites and different posts about this topic - but no where a final solution.

Thanks for any little help getting this done - appreciate it!