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The proper method is the enable the [Change Status] button on the workcenter, and use it to modify the job (operation) status from Held to Started.

The quick, dirty and easy method is the use a SQL statement:

UPDATE ManufactureOrderOperation
SET OrderStatusId = 2 -- 2 = Started
WHERE OrderStatusId = 4 -- 4 = Held
    and SoftDeleted = 0