If you allow logged in users to edit their web user account's username and password, you can customize the message displayed after they have done so:
- Go to the Salesforce User Manager
- Click the Registration Options tab
- For Message for Successful User Edit, enter the text you wish to display to the visitor
- Click Save
NOTE: This message is only displayed when the web user account information is edited. It is not displayed when the Contact record associated with the web user is edited. That message is customized on the Options tab under Submission Messages of the Forms page used for editing the record.