I accidentally claimed the wrong child how do I put on the right one?

If you already filed your tax return you need to wait until the return has been fully processed and then you can amend to change the dependent you claimed.   If there is someone else who can claim the child that you incorrectly entered on your tax return, they will have to file by mail.   You put the child's Social Security number into the system already and no one else will be able to e-file using that SSN.   The person who has to paper file can expect a wait of 6 to 9 months for the IRS to process a mailed return.   They do not have to wait for you to amend; it will not help them to e-file.

https://ttlc.intuit.com/community/tax-return/help/how-do-i-amend-my-2021-return/00/2544441