You'll find that the Apex slicer software requires you to be in the quickprint settings to change the nozzle.  You will be unable to specify nozzle size in advanced mode.  This is because that when you change the nozzle size, there are a whole host of other adjustments one needs to make to certain settings relating to the first layer.  To ensure these adjustments are made, the quickprint defines the nozzle and affiliated first layer settings.  From there, you open advanced mode to make adjustments.