Yes, you can amend your return to remove a dependent. Go to the following link for instructions to amend a 2018 tax return that you filed with TurboTax Online.
An amended return has to be filed on a special form, Form 1040X. It cannot be e-filed. You have to print it, sign it, and file it by mail. It takes the IRS up to 16 weeks to process an amended return.
Don't file the amended return until your first return has been accepted and you have received the original refund.
If you are removing the dependent so that someone else can claim the child, the other person will not be able to e-file their tax return, even after your amended return has been processed. They will have to print their tax return, sign it, and file it by mail.