A task inherits the workstate from the calendar in which it is stored. Therefore, to change a task workstate, "send" it in the calendar with the appropriate workstate.
Select one or more tasks, and then on the File menu click Send. Select the calendar and click Ok.
An alternative is to drag tasks to the desired calendar on the tree (Tasks navigator of the Planning main tab).
On the Tasks navigator of the Articles main tab you can right click a task and then select "Change workstate".
See Creating a calendar copy flow.
See also
Note
To be able to change a workstate of a task, there must exist a calendar copy flow. See Create a calendar copy flow for instructions on how to create required copy flow.