I am sorry for your loss.

You can amend your 2015 tax return and include your child as a dependent. Sign into Turbo Tax. Select My TaxTimeline. Select 2015. The, you will select Amend a Return from the list of things that you can do.

See full instructions in the article below.

https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed