it's deductible

Not quite sure what you are trying to do.  If you donated to charity in 2020 those donation will go on your 2020 tax return when you prepare it next year in 2021.

If you already filed your return, you have to amend it to make any changes.

If your return is accepted, wait until your return has been fully processes and you received your refund ( if you are expecting a refund ), then amend your return. 

Here's how you amend your return:

1. Sign in to your account.
2. Scroll down to Your tax return & documents , select 2019 and then Amend (change) return. 
3. Select Amend using TurboTax Online.
4. Follow the instructions, your refund or tax due will change to 0. This is normal on amended returns.

Amended return can't be e-file, you need to print it and mail it in.