When preparing an amended return, you'll use Form 1040X. On line 16 of that form (linked below), you'll see where to enter the amount you paid with the original return.
https://www.irs.gov/pub/irs-pdf/f1040x.pdf
Here's the TurboTax FAQ for amending a 2018 return.
https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/01/27579
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