If you booked together, the price will have to be the same for everyone on the confirmation code. Ie if you book 2 people together, you'll have to find 2 tickets with lower price.
To get the credit, got to "Change Flight". Go through the process as if you're changing your flight. Pick the same flight you've been booked and it'll charge the new price and apply your old flight's credit. The remaining credit will be under the same confirmation code, tied to the same passenger, and expire 1 year from when the original flight was purchased.