How do I rotate mesh and armature that already have animation key

My model facing the wrong direction. Its already fully animated.
How do I rotate it from facing north to south ?

Easy way if your just doing a walk cycle or what not with no actual set, you can just rotate the camera 180 degrees around the character.

Fastest way for armature animation, is to just select the root bone and rotate it to the desired direction and then apply a key frame(s) to it.

If you already have a key frame assigned to the root in that direction, just insert another one over each key frame you have in the dope sheet.

Edit: clarified a few things