Change arms holding animation in basic FPS level?

I want to change the way the player character’s arms animation so it doesn’t hold his arms out for the weapon that I will not be using in my project. I don’t want to remove the arms, I just want them to either do a standard running animation, or even just no animation at all, as long as they cannot be seen in game from the FPS perspective.

How do I change the animation on the skeletal mesh of the arms to:
A) Nothing
B) A simple running animation

How can I do that in a fresh FPS project using Starter Content?