Consider that not all arguments in the exact same format are valid and with a true conclusion, even if their premises are true.
Obama was an american president or Arnold Schwarzenegger was an American president.
Obama was an American president.
Therefore, Schwarzenegger was an American president.
Think of classical logic as a set of allowable operations, like mathematics. A or B, A, therefore B is not one of the allowable operations, nor is it derivable from any allowable operations.
how is it that we can change this argument to be valid?
That really depends, the way you asked this question implies you think there's some kind of valid argument in there.
Here's one possible valid argument.
A and B. Therefore B. If you change the or to an and, the conclusion follows.