You have 2 options: remove the yahoo account sync (in "Accounts & sync") or in the apps manager you can clear data for Yahoo mail app.
Thanks Dan. I just ended up uninstalling and re-installing the app (about 60 seconds). Not sure if they have updated or changed it but now with the reinstall under the folder view there is an accounts link that goes to manage accounts.
The colors have changed too. In doing research via google on this there were many folks complaining about the no log out feature. I guess in the past Yahoo has resolved the problem by telling people to feature. Does not seem to make any logical sense to have logging out of an app function like that. I will try your suggestions next time.