File a change of address with the post office and have them forward your mail to your new address. You should also mail a Form 8822 Change of Address to the IRS, but it probably will not get processed before they send the letter about the refund.
It's unlikely that you will have to return any money to the IRS. In the unlikely event that you do have to, as I said, the letter will tell you what to do if you think it's wrong. If you make a payment, either online or by check, that the IRS doesn't think you owe, they are just going to send it back to you.
