I'm guessing you want to override the caption of a display element, such as Label, Button, etc..
If that is the case, add a text element with the name <Caption> to the display. To change the display's caption dynamically, set the <Caption> element to whatever value you wish.
A trivial example, in which label A will actually display the caption B, can be found in the following screenshot:
For best results, use the Firefox browser..