If you enter your dependent(s) and your income earned from working the child tax credit is added automatically if you qualify--you do not have to "change" to it.
https://ttlc.intuit.com/questions/1900923-what-is-the-child-tax-credit
Are you only getting the $500 other dependent credit, instead of the $2000 child tax credit?
This usually means one of two things; 1. the child is over 16 or 2. he does not meet the rules for a "qualifying child" dependent.
Your child may be too old (over 16). You can still claim your child, as a dependent. What you can't claim is the Child tax credit. This comes as a big surprise to many parents the year their child turns 17. A child over age 16 no longer qualifies for the Child Tax credit (CTC). Although a child can still be a student dependent through age 23, and a qualifying child for EIC, the Child Tax Credit expires the year they turn 17 and you no longer get the $2000 CTC. Instead you will get the non-refundable (up to) $500 Other Dependent Credit.
The child must be closely related to you. The child must be your own child, a stepchild, or a foster child placed with you by a court or authorized agency. An adopted child is always treated as your own child. ("An adopted child" includes a child lawfully placed with you for legal adoption, even if that adoption is not final by the end of the tax year.) You can also claim your younger brother or sister, stepbrother, stepsister. And you can claim descendants of any of these qualifying people—such as your nieces, nephews and grandchildren—if they meet all the other tests. The child must live with you for more than half the year.
See full rules at