Like I'm specializing in combat, but I wanna specialize in magicka now.

Console command is enableclassmenu
Change your minor and major skills as you like.
Save the game as a new save.
Exit the game completely.
Start it up and load your new save game.
Right click to check your new settings.

If you don't close the game completely or you check your menus before closing the game down then you won't see your new choices.

Thanks!