It was one of the UI features that the players used to ask for:)
Click on your avatar on the main screen. You will see the "Amnesia" button.
Click on it and you will start the campaign from the very beginning. You will also be able to replay all the side quests! You will need 1 elorite to perform the Amnesia ritual.
I have added side quests for the first location recently so I encourage veterans to use Amnesia too.
Check the first location again and try the new PvE experience. That's how all the locations will work soon :)