Changing a Seletal Mesh doesnt change ingame

I want to edit the basic Skeletal Mesh (Screenshot by Lightshot)
All I wanted to do, is changing the skeleton’s arms a bit, like this: Screenshot by Lightshot
After I changed the position of the skeleton’s arms, I pressed save and closed the tab.
When I get back to my Scene window, nothing changed and when I want to change the skeleton again by double clicking on the Skeletal Mesh window, it’s back at it’s original position. Screenshot by Lightshot
What am I doing wrong?