If you have not yet filed your tax return you can add your wife by signing into your Online account and going to the Personal Info tab at the top. You can then change your filing status and add your wife.
If you have already filed your tax return you would first need to wait and see if your return is accepted or rejected. If rejected, you can go back and add her following the steps above. If accepted, you would have to wait until you have received your tax return and then Amend your tax return using Amendment Form 1040X.
Here's more information on Amending: