Modify Default Mannequin Animations

As the title states, I am trying to modify the existing animations that come with Third Person in Unreal Engine. However, I don’t really find a way to do it in Unity. When I open the default animation, I get a blank keyframe editor as shown in the below screenshot

I am new to Unreal and will appreciate any help on this.

Thank you so much for the link. This clears things.