if I set the fee too low how will I fix it?
You have a few options here. You can wait and hope it confirms, wait for the transaction to be "forgotten" by the network, attempt a double spending transaction that pays a higher fee, or ask a miner to confirm it for you.
For the third option, there should be something in Multibit that is like "reset wallet" or "repair wallet" which would clear all of the unconfirmed transaction from your wallet and allow you to resend the transaction but with a higher transaction fee.
For the fourth option, you can contact the users
Quickseller and
macbook-air. You can also try using
https://www.viabtc.com/tools/txaccelerator/ to "accelerate" your transaction.