That CTC increase is not for 2020; it is for 2021. Your 2020 return does not have to be amended for it. More information will be available soon; the details of how it will be implemented have not yet been clarified fully. There will be monthly payments going out beginning in July 2021, with some of the CTC still left as a refundable credit on your 2021 tax return next year.
if the child is a newborn in 2021, you'll have to register the child to get the advance payments. the IRS has not yet opened a portal to do so but check their webpage for further info.