It may take some time to get a hang of, but its really not that hard if you think about it. Once you have something to reference from you can then modify the numbers and set a certain objective to 'Complete' some missions may be different though whereas 0 Is incomplete and 1 is complete - remember that! Image below will give assistance if needed. Like I said before you will need BL2 or a wiki to reference the 'complete' number of the mission requirement. You can access these options by looking to ObjectiveProgress and SubObjectiveSetIndexes opening one of these will show a set of numbers depending on the quest. If you want to complete a few sub objectives, or objectives you will need BL2 to reference the number of requirements. Once you've found the mission you want to edit look to Status and change the mission to ReadyToTurnIn if you want to instantly complete the mission. There are two ways to look for the quest you REALLY want to edit, #1 is looking to mission on the right this will show you the name or 'Codename' of the mission, and #2 is seeing if the mission is Active, Not Started, or Failed. Once you're in your quest information you will notice a lot of quests. Once you've clicked one of these navigate your way to the MissionData collection arrow. Depending on what playthrough you're on, choose from 1 of 2 existing options available since Playthrough 3 has no data yet.

Now that you've opened up the collections list you will notice 3 MissionPlaythroughData the first one is Playthrough 1 (Normal Mode), the second one is Playthrough 2 (True Vault Hunter Mode), and the third one is most likely the 'Something Interesting thing' that Gearbox Is planning on when they raise the level cap which will be a Playthrough 3 no doubt. Image below will give assistance if needed. When you find, it navigate your way to the arrow that will open up the collection window. Now scroll down until you see MissionPlaythroughs. Firstly you will need to open your save into Gibbed's Borderlands 2 save editor and move your way to 'Raw' to edit your characters Raw Data.

Gibbed save editor borderlands 2 guide