This will be a super-short tutorial on how to load and modify a saved animation in Blender. Let's say you are going to make an animation for Unity. You have a character and maybe two animations, one where the character is doing nothing except moving its head, and one where the character is walking. You did the "doing-nothing-animation" first and then you did the walk animation.
What if you came up with the brilliant idea that the "doing-nothing-animation" should also include an animation where the character is moving its arm. So you want to make a simple modification to that animation. You select the animation, or action which is the name of a saved animation, in the Dope Sheet window. But you will now discover to your horrors that the character is still walking, which is the latest animation you made.
What you need to do is to first select all bones and then open the animation.