You can claim your daughter if she meets the guidelines posted at the bottom. The most important thing is that if SHE files a tax return, she must mark that she is being claimed by someone else.
You would just need to amend your return and include her as a dependent. Here's the link to the instruction (select 2018)
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
Qualifying child
- They are related to you.
- They aren't claimed as a dependent by someone else.
- They are a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
- They aren’t filing a joint return with their spouse.
- They are under the age of 19 (or 24 for full-time students).
- No age limit for permanently and totally disabled children.
- They live with you for more than half the year (exceptions apply).
- They didn't provide more than half of their own support for the year.