Whether you only use the built-in ringtones, or your have added more (details below), it's easy to change from one ringtone to another

Changing the ringtone for Phone, Messaging or Calendar
The built-in applications have a one-stop shop for switching tones.

Press Applications Launcher .
Launch the Prefs application.
Under the General heading, select Sounds & Alerts.
On the Sounds & Alerts preferences screen, choose an application from the pick list at the top of the screen.
To change the tones that play, tap on the Tones box near the top of the screen for each application.


Zolved - Ringtones: changing, adding and managing on a Treo 700p or 650 smartphone

How to change ringtones such as adding or deleting on Treo 650?