Change from ITIN number to SSN to get Child credit
No, you can not update a filed return. You will have to file an amended tax return to change your child's ITIN to their SSN.
The link below has information on amending tax returns that you may find helpful.
https://ttlc.intuit.com/replies/3288565
Is this for 2018. or an earlier year?You may not claim the credit on either your original or an amended return for a child who doesn't have an SSN by the due date of that return (including extensions), even if that child later gets an SSN.
<a rel="nofollow" target="_blank" href="https://www.irs.gov/faqs/childcare-credit-other-credits/child-tax-credit/child-tax-credit-4">https://www.irs.gov/faqs/childcare-credit-other-credits/child-tax-credit/child-tax-credit-4</a>