You have two choices. You can file an Amended return and report the Form 1099-DIV and pay any additional tax; or, you can wait for the IRS realize you didn't include the 1099-DIV on your tax return. for the later, the IRS will propose a change to your tax return and bill you for any additional tax plus any applicable penalty and interest. If the amount of the dividend is small, I would wait for the IRS.
Do I reenter everything that was on my previous return? I already received my over payment...
When you start the amendment process, you should open the TurboTax return that you already filed. Indicate that you are going to amend the return (In fact, I don't think you can do most things do the return until you agree). Then add the 1099-DIV. You should not change any other entry.
TurboTax should generate a new form - 1040X.
You will need to print and mail the 1040X and any tax return pages that changed.
To learn more information on how to amend your return, please see this TurboTax FAQ.