I inadvertently entered a prepayment on my tax return. Now it's showing a credit. How do I delete the prepayment credit from my record?

Was it entered as an Estimated payment, check 1040 line 26.  Or as an Extension payment on 1040 line 31?  

If you need to delete an Extension payment,  see this Extension FAQ for how to get there.  Then you can change to amount to zero.

https://ttlc.intuit.com/questions/3205781-how-do-i-get-credit-for-tax-payments-i-made-with-my-extension

You can type estimates paid in the search box at the top of your return and click Find and it will give you a link to Jump To the screen where you enter Extension Payments.

To enter, change or delete a payment made with an extension request (Federal, State, Local) -

• Click on Federal Taxes (Personal using Home and Business)

• Click on Deductions and Credits

• Click on I'll choose what I work on (if shown)

• Scroll down to Estimates and Other Taxes Paid

• On Other Income Taxes, click on the start or update button

On the next screen select the type of extension payment made and click on the start or update button