In your web page, click the text that you click for the popup to happen.
You will then see the behaviour listed in the Behaviours Task Pane; if the
Behaviours pane is not visible, turn it on from Format > Behaviours....
In the Behaviours Window/Task Pane you will see all the behaviours you have
in your site, among them will be this one. Double click it and it will open
the "edit" window for you to edit the text for the popup/alert box.
Else, you can go to code view and edit the text in the parentheses. where
is says <p onclick......
Hope this helps.
Andrew