I'm not using Chessbase 12, but rather the older 2009 Premium. Anyway, the setting must be the same: under options-- misc untick the box "always promote to queen" (I guess you meant queen, not king- which is rather illegal... :P).
Pawn promotion in Chessbase 12
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Promotion to a king could be a good defence - your opponent would have to checkmate both of them 
Jan 13, 2014
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#5
Thanks for the help. Your correct; go to options, misc. and change the setting. Fantastic!
Even switching clear still promotes to Q. Any solution for this issue?
I know it is a very old topic but I had the same problem. When I was searching for the solution, I found this topic. However I later found the solution myself. I'm going to write it here for people who are going to have the same problem in the future. 
You have to go to the "board" section (that is where you choose the view of the board, the pieces, etc.). There you can find an "always promote to queen" button and you can cancel it 
Chessbase 12 assumes that all pawn promotions are made to a king, however, I have a couple of games where the pawn was promoted to a knight and rook. How can you change the promotion in this program? I'm sure it can be done some way. Thanks