Did you file a 2017 return and want to amend it to add a dependent? Or are you preparing 2017 now? Did you use the Online version or the Desktop program?
How to amend 2017. You will need to install the Desktop program on a full Windows or Mac computer.
https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/00/27372
