Here’s how:

1. Click the Messenger menu and select Preferences.
2. Click the "General" category.
3. Click the "Edit mobile device" button and enter your Yahoo! ID and password to sign in to Yahoo! Mobile.
4. Under the Other Mobile Devices section, click the trashcan icon to the right of the device.
5. Click Delete.

Note: To change your Go Mobile device, you must first delete your original device and then register your new device.