How do I unlearn words?
Click on the right-arrow ">" at the top of the keyboard.  This will pull up a list of words.  Long press on a word and it will say "___ will be removed from learned words". Then press OK and it's gone!

Unless you are trying to remove an email address; then it doesn't disappear! :(  Really annoying.  Anyone know how to fix that?

On a Samsung Galaxy Note 4.