StarCraft 2 Continuing Campaign
If you press "Continue Campaign" after finishing the last mission, it will bring up a window similar to the Missions Archives you could access during the game. For all of the missions where you could choose one mission or the other, both options will be available to play here.
The only mission that you can't play if you missed it is the Secret Mission. To do that, you'll either need to start a new campaign or load a save game from before you finished the mission where you unlock the Secret Mission.