How do I answer a previously submitted question?

Click on the question, and then type in the box that says "Add your answer " if there is one. If there is no box then the question is closed to new answers or the user who asked it has you blocked.

Click in the rectangular box, then type away.

Same way as you would if you were to Answer someone Else's Question go to the Question click on the Add your Answer Box type your Answer in and Click Submit

If you can find the question, you should be able to add yiur answer, even if a Best Answer has already been awarded. If you can't find the question, Yahoo has probably deleted it.