Double click the dimension, with the mouse over the selected dim RMB to bring up the context menu; at (or near) the bottom of the dialog will be an option to "Convert to automatic".
In the drafting application go to the menu Edit -> Annotation -> Text, now select your dimension and overwrite the dimension text with what you want. A dialog may appear warning of the evils of overriding dimension text; if you are sure of what you are doing click OK.