Hi, I too am a new user with yahoo account. I downloaded the yahoo app and found that once into my emails, there is NO facility to log out. I googled and found hundreds of people who also discovered this. I removed the yahoo app via settings.
I then google yahoo mail and entered my email address and password and entered, there was the log-out feature as per laptop/pc.
I too experienced yahoo telling me everything was wrong, but after following the above, I am sure you will have success.