You can modify your missions directly from your Business space.
If a mission is not yet staffed, you can modify it by clicking on the specific mission, then “Modify mission”. You can then access all the criteria you have provided to modify them (job, dates, hours, equipment...).
However, if a mission has already been staffed, you will not be able to modify it but you will be able to indicate if it has lasted longer or shorter than excepted by clicking on “Modify duration”.