To Display or Edit a Flag's Explanatory Message from Selected Flag

Select the flag(s).
Select the Show/Edit Flag tool.
The Define Flag Information dialog opens.
(Optional) In the dialog, edit the message text.
When done, click OK. If you selected more than one flag, return to step 3; flags are processed in the order in which they were selected.

You can also select the Show/Edit Flag tool first. Then select the flag. Then accept the flag. The Define Flag Information dialog opens.