How to rotate my character's skeletal mesh in UE4?

As above. I wanted to add a Mannequinn and rotate all the bones like it was a sitting man. The problem is when I save those changes just nothing happens. I mean, when I use UE4 for it e.g. for changing my Skeleton or Mesh in it’s editor mode - nothing happens when I save it. How can I set it like that and how can I rotate bones so I can fit them with a gun?
I’d like my character to hold it’s weapon always up and running, jumping and so on in one pose. How to?