You can do so by pressing on the mission briefing screen. To start, you can customize your loadout before starting the mission. You are going to assist the Savimbi with eliminating the MPLA and rescue Woods. This mission takes place back in the 1980s.
I would suggest only using this if you failed to make a decision that will cause you to not earn an achievement in later missions. You will have to find all intel you may have collected and redo all challenges for the missions as well. Once you do this, you will have to play through all missions after this point even if you have already completed them. In the campaign, there is a rewind story feature that allows you to go back to a select mission and change a decision you may have made. It will wipe all of the challenges you've completed, all of the intel you've acquired, and all of the decisions you've made. DO NOT SELECT NEW STORY FROM THE CAMPAIGN MENU. If you happen to miss a few challenges in a mission, you can choose to replay that mission to complete the last challenges you may have missed. While playing through each mission, when you complete a challenge, a message saying "Career Record Updated" will show up in the top left corner indicating that you completed a challenge. Once you completed all challenges, you will earn the 'Giant Accomplishment' achievement. You can view the challenges before the start of each mission by hitting and then selecting a specific mission. One achievement will require you to complete all of these. For each of these missions, there are 10 challenges for 160 challenges in total. These missions will affect how some situations proceed depending on whether or not you complete them or leave them undone. There are also 5 missions called 'Strike Force' missions. There are 11 mandatory missions that need to be completed. YOU CAN PLAY THESE STRIKE FORCE MISSIONS ON RECRUIT AND YOU'LL STILL EARN THE VETERAN ACHIEVEMENTS IN THE END, JUST MAKE SURE YOU COMPLETE THE STRIKE FORCE MISSIONS WHILE YOU CAN BEFORE THEY DISAPPEAR. NOTE: IF YOU'RE PLAYING THROUGH ON VETERAN, MAKE SURE TO PLAY THE STRIKE FORCE MISSIONS AS SOON AS YOU CAN. It may be recommended for you to search the loadout you prefer most, but things like Target Finder, the Storm PSR and the Toughness perk will always come in handy. Also, after you have finished the campaign once you will have more choices in weapons to use. When playing through on Veteran difficulty, you can simply choose to replay a mission and put the difficulty on Veteran.
After you tackle the challenge, collected all the intel and make some of the game altering decisions, play through on Veteran difficulty for the last remaining achievements. I suggest playing through the missions on Recruit difficulty to ensure you can complete all mission challenges. The campaign will more than likely require 2 playthroughs to complete all achievements.
You will play missions that happened in the 1980s along with futuristic missions from 2025. The story of Black Ops II incorporates some of the main characters from Black Ops I along with incorporating some from the previous (Treyarch) game in its storyline.