I have previously assigned a cell phone number to my messenger and now would like to change it - but how?

With the messenger open, in the drop-down box ';Messenger'; there is an option titled ';Sign in to 639205347206.'; I no longer use this number and would like to change it to my current cell phone number but the option is no longer available. How can I change this number?I have previously assigned a cell phone number to my messenger and now would like to change it - but how?
open yahoo messenger --%26gt; go to Messenger tab(top left) click on that u will get a dropdown menu --%26gt; click on Preferences--%26gt; go to General tab --%26gt; goto EDIT MOBILE PHONE



u can change there or delete the no