My daughter is 17 which didn't qualify for the child tax credit. I filed my return early. If Biden changes the child tax credit to include 17 year olds, can I ammend?

If, and this is a big

IF

, the plan is approved by Congress, signed by the President and made retroactive to tax year 2020, then you would have to file an amended tax return for 2020.  Or there may be other ways to receive the tax credit.   OR there may other ways to claim the credit, you will just have to wait and see.

And by the way, big update....

IRS won't open filing until Feb 12, 2021 this year!

https://www.irs.gov/newsroom/2021-tax-filing-season-begins-feb-12-irs-outlines-steps-to-speed-refunds-during-pandemic