I clicked yes to the "First-Time Charitable Donor Super Credit", but I may have claimed donations in the past. Where can I uncheck that box? I can't find it anywhere.

When using TurboTax Standard, click on the heading titled Setting up and scroll down to Personal Tax Profile. Click on edit next to Other credits & deductions.  Scroll down to the question, did you make donations to a charity or political party? Under that you will see the question, I made donations for the first time that qualify for the First-Time Donor Super Credit.  Make sure to remove any checkmark that you might have next to it.