To replace labels already displaying with new ones, do not delete the old ones first. You will get unexpected results if you do this. For example, if you create labels with an expression and want to change the expression, do not delete the labels first. Simply change the expression in the Label Display tab of the Layer Properties dialog box. If you delete the labels first, you are deleting the label for the object, not a piece of text. So, if you create labels with a different expression, they will not display because by deleting the old labels, you have edited the layer so that it does not display labels for these objects. On the MAP tab, in the Label group, click Clear Custom to revert back to the default settings.