You could pass a pointer to the logic class just like any other variable, but I wouldn't recommend directly accessing the IBOutlet property.
What I'd recommend, is either having the logic class return the values and have the controller update the label as needed, or if it involves background processing that doesn't return immediately, use the delegate pattern. This way, the logic class will inform the controller when the data is ready, or the calculations are finished, and then the controller can update the UI as needed.
Look into iOS Protocols to define the structure of a delegate class :)