how do you change the cell phone on yahoo messenger?

To change your Mobile number you must first delete your original device and then register your new number:

Click the Messenger menu and select Preferences.

Click the "General" category.

Click the "Edit mobile device" button and enter your Yahoo! ID and password to sign in to Yahoo! Mobile.

Under the “Other Mobile Devices” section, click the trashcan icon to the right of the device.

Click Delete.

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Here’s how to edit/update your mobile device info:

Under the "Other Mobile Devices" section, click the pencil icon to the right of the device. Edit any of the following information:

Device's Make and Model

Device's Name

Daily Message Limit

Click the "Change" button to save changes.

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On Messenger go to My Contact Details and replace your mobile number with the new number.

Click Save & Close.