How do I change my federal tax payment to show up as a payment toward my 2020 taxes instead of 2021?

I can help you get this transaction set to the correct year, @infoabbyrose.

As a note, you can always consult your accountant if you want more tips on how to best record this.

You'll want to delete the tax payment and enter it again in your QuickBooks Self-Employed (QBSE) account. Here's how.

If the tax payment is a downloaded transaction from your bank, you'll have to exclude it and manually enter it. Let's locate the payment and exclude it.

1. Go to Transactions.
2. Filter the list of transactions to show This Year only.
3. You can also filter the Category Type for Estimated Taxes only. 
4. Click the drop-down arrow to expand transaction details.
5. Put a checkmark on Exclude. 
6. Hit Save.  

Once done, here's how to manually add it:

1. Click the Transactions tab.
2. Select the Add Transaction button.
3. Enter the Transaction Details. 
4. Make sure to use dates within 2019 as the date of the Transaction.
5. Click Save.

Here's an article you can read for more details: Manually add transactions in QuickBooks Self-Employed.

However, if the payment is an imported transaction, you'll want to exclude it then import it again. Before importing it to your QBSE account, ensure to edit the date of the transaction. For reference, you can check this great article: Add older transactions to QuickBooks Self-Employed.

Please feel free to reply if you have additional questions Take care!