How to continue playing with any selected mission and? (Call of Duty: Black Ops 2)

and how to see the statistics in the game ??? In general, I passed it on a veteran, but the only drawback is that I did not have time to save karma, in general, the ending did not satisfy me, I decided to replay the mission with karma, everything went well, but how now to continue playing with this mission ?. and finally explain the principle of the saves in the game, otherwise I won't understand them in any way, everything is simple in the stalker, you look for any mission and start the further passage from it, but how?