Go back through My Information. You'll come to a little table showing your dependents. Choose whichever one you want to edit and make your changes in that interview.
If yu've already filed your return, you'll have to wait to do this in an amended return. Amended returns are filed by mail and it can take the IRS up to 16 weeks to process them. If your original return is rejected, make your changes and refile. If accepted, wait until you get your refund before you file the amended, as instructed in this link: