I assume you mean for a password-protected section in OneNote? Te
procedure is the same in both 2003 and 2007.
Navigate to a password protected page, and unlock it (by clicking on the
page then typing your password). Open up the Task Pane from the View menu
and then select the Password Protection Task Pane.
From this Task Pane, you must "Remove Password" and then "Set Password"
with the desired new password.