You should file one amended return that lists all your correct information. If you have more than one thing to correct you must include everything.
We can't see your tax return. There are three documents you should print out and review.
1. Your original tax return
2. Your new tax return
3. the 1040-X that comes with your new tax return.
First, review your new tax return to make sure it is accurate. Compare with the old return if needed. Then, look at the 1040-X, which is a summary of the changes. It's basically just an addition/subtraction problem with lots of rows, you should be able to follow the math.
For 2020 only, adding a child dependent could be worth at least $3100, possibly more if you are low income and qualify for EIC.
However, it may be useful to know why you are adding a child now, and why you did not claim them before. There might be reasons why the IRS would not pay out for a new dependent.
