Here we go,
Step 1 . Open Your Yahoo Messenger and Go Choose Messenger-->Privacy Options
Step 2 . Now Choose (1) General, and then (2) Clear the Check Box of " Always sign into my mobile device when I exit Messenger", After that (3) Click on Edit Mobile Device.
Step 3 . Now Sign in your yahoo account within your yahoo user name and password.
Step 4 . Now You will find TWO options Edit or Delete, if you want edit your mobile number then press edit and change your mobile number and save and close, other wise Press Delete icon button.
Step 5 . Now you will find DELETE DEVICE button, press that button and your work is complete.