Did you already file a return for 2022? If yes, then you need to file an amended return to change from the ITIN to the SSN, in order to qualify for the larger child tax credit and possibly other benefits that require an SSN. The amended return should not be filed until your first return is processed and any refund paid, but the amended return MUST be filed before the April 18 deadline. If you can't file by the April 18 deadline, you need to get an extension and then file by October 15.
If you have not filed yet, and are having trouble changing your child’s info, the simplest thing to do is delete the child from the list of dependents and enter them again with the revised information.
You may not be able to efile your new return (if you are filing a new return) or your amended return, if the IRS doesn't recognize the new SSN. In that case, mail your return, using a mailing service that has tracking and proof of delivery.
Separately, you need to send a letter to the IRS as described. But sending the letter by itself, will not change your tax return or get you any extra credits.
