This response came directly from the HUD website:
Can I modify an offer after it has been submitted?
Yes you can modify an offer after it has been submitted up until the bid-opening date. Log in to HUDHomestore.com, click Bidder Functions, and go to Review your Bids. Click the bid you want to cancel. Click the Modify Bid button. A summary screen will appear where you can modify the bid. Click the Continue button. Review the bid summary, click the check box to accept the terms and conditions, then click the Confirm Bid Submission button. The modified bid is then submitted and the original bid is withdrawn from consideration by the Asset Manager. You are not able to modify an offer after the bid-opening date has passed.