How do I amend this years taxes before I get them back ? I claimed bot of my grandchildren for living with me but I need to let there dad claim one of them.

You can not amend until the first return is fully processed and you get the first refund.  And the amended return won't be ready until March.

The dad will have to mail his return this year to claim them.  Even if you amend he has to mail.  So he doesn't have to wait.  He should mail his right away.

Amend a 2020 Online return

https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/00/

No you cannot claim a child for part of a year.   You either claim the child for the whole year or nothing.

IRS interview to help determine who can be claimed:

https://www.irs.gov/help/ita/who-can-i-claim-as-a-dependent

https://ttlc.intuit.com/questions/3113432-who-can-i-claim-as-my-dependent  

If you should not have claimed the child(ren) then you will need to amend AFTER your return has been fully processed.  The Form 1040X is not available until the end of March.   If the child's father is claiming the child then he has to print, sign and mail his own return now--he cannot e-file.   Your amended return will never "catch up" in the system so he cannot wait for that and e-file---he has to mail his return if he is claiming the child.