The planning board can be found at Backoffice -> Dispatch: left frame.
(click to enlarge)
To un-assign a job from a node just 'drag' the triangle to the [remove] button in the right upper corner of the left frame. The job is not removed from the cargoOffice but shows up in the right frame again. It can be re-assigned to a different day or node.
If the job is part of a trip then the whole trip is un-assigned and either the trip or the individual jobs will show up in the right frame again..