You can edit your purchase (including quantity, billing information, or canceling your order) until midnight the day of the deal, or while the deal is still live.
To change your purchase, please sign into your account at www.groupon.com. Once you are signed in, you will see your name at the top right corner of the deal page. Place your cursor over your name and your account menu will drop down for you to access "My Groupons".
From "My Groupons", click on "Edit order" next to the Groupon you purchased. On this page you'll be able to make changes to your order: you can change the quantity, update the billing info, or cancel the order completely.
Jun 20, 2013