When a mission is published, only it's title, image, mission description and audience can be edited.

You may need to edit more than these factors like the reward. To save time of re-creating a mission, you can simply 'copy' the mission so a duplicate mission will appear in draft stage by clicking 'Mission Options' on the mission and choose 'Copy Mission'. 

See here for video reference: https://cl.ly/d7b1d8d1c2de

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